Actin and myosin contribute to mammalian mitochondrial DNA maintenance.

Mitochondrial DNA maintenance and segregation are dependent on the actin cytoskeleton in budding yeast. We found two cytoskeletal proteins among six proteins tightly associated with rat liver mitochondrial DNA: non-muscle myosin heavy chain IIA and ß-actin. In human cells, transient gene silencing of MYH9 (encoding non-muscle myosin heavy chain IIA), or the closely related MYH10 gene (encoding non-muscle myosin heavy chain IIB), altered the topology and increased the copy number of mitochondrial DNA; and the latter effect was enhanced when both genes were targeted simultaneously. In contrast, genetic ablation of non-muscle myosin IIB was associated with a 60% decrease in mitochondrial DNA copy number in mouse embryonic fibroblasts, compared to control cells. Gene silencing of ß-actin also affected mitochondrial DNA copy number and organization. Protease-protection experiments and iodixanol gradient analysis suggest some ß-actin and non-muscle myosin heavy chain IIA reside within human mitochondria and confirm that they are associated with mitochondrial DNA. Collectively, these results strongly implicate the actomyosin cytoskeleton in mammalian mitochondrial DNA maintenance.

Myosin IIB is required for growth cone motility.

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

A Rho-dependent signaling pathway operating through myosin localizes beta-actin mRNA in fibroblasts.

BACKGROUND: The sorting of mRNA is a determinant of cell asymmetry. The cellular signals that direct specific RNA sequences to a particular cellular compartment are unknown. In fibroblasts, beta-actin mRNA has been shown to be localized toward the leading edge, where it plays a role in cell motility and asymmetry. RESULTS: We demonstrate that a signaling pathway initiated by extracellular receptors acting through Rho GTPase and Rho-kinase regulates this spatial aspect of gene expression in fibroblasts by localizing beta-actin mRNA via actomyosin interactions. Consistent with the role of Rho as an activator of myosin, we found that inhibition of myosin ATPase, myosin light chain kinase (MLCK), and the knockout of myosin II-B in mouse embryonic fibroblasts all inhibited beta-actin mRNA from localizing in response to growth factors. CONCLUSIONS: We therefore conclude that the sorting of beta-actin mRNA in fibroblasts requires a Rho mediated pathway operating through a myosin II-B-dependent step and postulate that polarized actin bundles direct the mRNA to the leading edge of the cell.

Structural abnormalities develop in the brain after ablation of the gene encoding nonmuscle myosin II-B heavy chain.

Ablation of nonmuscle myosin heavy chain II-B (NMHC-B) in mice results in severe hydrocephalus with enlargement of the lateral and third ventricles. All B(-)/B(-) mice died either during embryonic development or on the day of birth (PO). Neurons cultured from superior cervical ganglia of B(-)/B(-) mice between embryonic day (E) 18 and P0 showed decreased rates of neurite outgrowth, and their growth cones had a distinctive narrow morphology compared with those from normal mice. Serial sections of E12.5, E13.5, and E15 mouse brains identified developmental defects in the ventricular neuroepithelium. On E12.5, disruption of the coherent ventricular surface and disordered cell migration of neuroepithelial and differentiated cells were seen at various points in the ventricular walls. These abnormalities resulted in the formation of rosettes in various regions of the brain and spinal cord. On E13.5 and E15, disruption of the ventricular surface and aberrant protrusions of neural cells into the ventricles became more prominent. By E18.5 and P0, the defects in cells lining the ventricular wall resulted in an obstructive hydrocephalus due to stenosis or occlusion of the third ventricle and cerebral aqueduct. These defects may be caused by abnormalities in the cell adhesive properties of neuroepithelial cells and suggest that NMHC-B is essential for both early and late developmental processes in the mammalian brain.

Conditional expression of a truncated fragment of nonmuscle myosin II-A alters cell shape but not cytokinesis in HeLa cells.

A truncated fragment of the nonmuscle myosin II-A heavy chain (NMHC II-A) lacking amino acids 1-591, delta N592, was used to examine the cellular functions of this protein. Green fluorescent protein (GFP) was fused to the amino terminus of full-length human NMHC II-A, NMHC II-B, and delta N592 and the fusion proteins were stably expressed in HeLa cells by using a conditional expression system requiring absence of doxycycline. The HeLa cell line studied normally expressed only NMHC II-A and not NMHC II-B protein. Confocal microscopy indicated that the GFP fusion proteins of full-length NMHC II-A, II-B, and delta N592 were localized to stress fibers. However, in vitro assays showed that baculovirus-expressed delta N592 did not bind to actin, suggesting that delta N592 was localized to actin stress fibers through incorporation into endogenous myosin filaments. There was no evidence for the formation of heterodimers between the full-length endogenous nonmuscle myosin and truncated nonmuscle MHCs. Expression of delta N592, but not full-length NMHC II-A or NMHC II-B, induced cell rounding with rearrangement of actin filaments and disappearance of focal adhesions. These cells returned to their normal morphology when expression of delta N592 was repressed by addition of doxycycline. We also show that GFP-tagged full-length NMHC II-A or II-B, but not delta N592, were localized to the cytokinetic ring during mitosis, indicating that, in vertebrates, the amino-terminus part of mammalian nonmuscle myosin II may be necessary for localization to the cytokinetic ring.

Calcium-dependent threonine phosphorylation of nonmuscle myosin in stimulated RBL-2H3 mast cells.

Stimulation of RBL-2H3 m1 mast cells through the IgE receptor with antigen, or through a G protein-coupled receptor with carbachol, leads to the rapid appearance of phosphothreonine in nonmuscle myosin heavy chain II-A (NMHC-IIA). We demonstrate that this results from phosphorylation of Thr-1940 by calcium/calmodulin-dependent protein kinase II (CaM kinase II), activated by increased intracellular calcium. The phosphorylation site in rodent NMHC-IIA was localized to the carboxyl terminus of NMHC-IIA distal to the coiled-coil region, and identified as Thr-1940 by site-directed mutagenesis. A fusion protein containing the NMHC-IIA carboxyl terminus was phosphorylated by CaM kinase II in vitro, while mutation of Thr-1940 to Ala eliminated phosphorylation. In contrast to rodents, in humans Thr-1940 is replaced by Ala, and human NMHC-IIA fusion protein was not phosphorylated by CaM kinase II unless Ala-1940 was mutated to Thr. Similarly, co-transfected Ala --> Thr-1940 human NMHC-IIA was phosphorylated by activated CaM kinase II in HeLa cells, while wild type was not. In RBL-2H3 m1 cells, inhibition of CaM kinase II decreased Thr-1940 phosphorylation, and inhibited release of the secretory granule marker hexosaminidase in response to carbachol but not to antigen. These data indicate a role for CaM kinase stimulation and resultant threonine phosphorylation of NMHC-IIA in RBL-2H3 m1 cell activation.

Nonmuscle myosin II localizes to the Z-lines and intercalated discs of cardiac muscle and to the Z-lines of skeletal muscle.

To understand the role of nonmuscle myosin II in cardiac and skeletal muscle, we used a number of polyclonal antibodies, three detecting nonmuscle myosin heavy chain II-B (NMHC II-B) and two detecting NMHC II-A, to examine the localization of these two proteins in fresh-frozen, acetone-fixed sections of normal human and mouse hearts and human skeletal muscles. Results were similar in both species and were confirmed by examination of fresh-frozen sections of human hearts subjected to no fixation or to treatment with either 4% p-formaldehyde or 50% glycerol. NMHC II-B was diffusely distributed in the cytoplasm of cardiac myocytes during development, but after birth it was localized to the Z-lines and intercalated discs. Dual labeling showed almost complete colocalization of NMHC II-B with alpha-actinin. Whereas endothelial cells, smooth muscle cells and fibroblasts showed strong immunoreactivity for NMHC II-A and NMHC II-B, cardiac myocytes only showed reactivity for the latter. The Z-lines of human skeletal muscle cells, in contrast to those of cardiac myocytes, gave positive reactions for both NMHC II-A and NMHC II-B. The presence of a motor protein in the Z-lines and intercalated discs raises the possibility that these structures may play a more dynamic role in the contraction/relaxation mechanism of cardiac and skeletal muscle than has been previously suspected.

Gene dosage affects the cardiac and brain phenotype in nonmuscle myosin II-B-depleted mice.

Complete ablation of nonmuscle myosin heavy chain II-B (NMHC-B) in mice resulted in cardiac and brain defects that were lethal during embryonic development or on the day of birth. In this paper, we report on the generation of mice with decreased amounts of NMHC-B. First, we generated B(DeltaI)/B(DeltaI) mice by replacing a neural-specific alternative exon with the PGK-Neo cassette. This resulted in decreased amounts of NMHC-B in all tissues, including a decrease of 88% in the heart and 65% in the brain compared with B(+)/B(+) tissues. B(DeltaI)/B(DeltaI) mice developed cardiac myocyte hypertrophy between 7 months and 11 months of age, at which time they reexpressed the cardiac beta-MHC. Serial sections of B(DeltaI)/B(DeltaI) brains showed abnormalities in neural cell migration and adhesion in the ventricular wall. Crossing B(DeltaI)/B(DeltaI) with B(+)/B(-) mice generated B(DeltaI)/B(-) mice, which showed a further decrease of approximately 55% in NMHC-B in the heart and brain compared with B(DeltaI)/B(DeltaI) mice. Five of 8 B(DeltaI)/B(-) mice were born with a membranous ventricular septal defect. Moreover, 5 of 5 B(DeltaI)/B(-) mice developed myocyte hypertrophy by 1 month; B(DeltaI)/B(-) mice also reexpressed the cardiac beta-MHC. More than 60% of B(DeltaI)/B(-) mice developed overt hydrocephalus and showed more severe defects in neural cell migration and adhesion than did B(DeltaI)/B(DeltaI) mice. These data on B(DeltaI)/B(DeltaI) and B(DeltaI)/B(-) mice demonstrate a gene dosage effect of the amount of NMHC-B on the severity and time of onset of the defects in the heart and brain.

Differential expression of non-muscle myosin heavy chain genes during Xenopus embryogenesis.

Class II non-muscle myosins are implicated in diverse biological processes such as cytokinesis, cellularization, cell shape changes and gastrulation. Two distinct non-muscle myosin heavy chain genes have been reported in all vertebrates: non-muscle myosin heavy chain-A (NMHC-A) and -B (NMHC-B). We report here the isolation of the Xenopus homolog of NMHC-A and present a comparative analysis of the developmental and spatial expression patterns of NMHC-A and the previously isolated NMHC-B to address the role of NMHCs in Xenopus development. A 7.5 kb NMHC-A mRNA is present, maternally in unfertilized eggs and throughout embryogenesis, as well as in all adult tissues examined. An additional 8.3 kb zygotic transcript for NMHC-A is also detected, but only during embryonic stages. Whole mount in situ hybridization with tailbud stage embryos shows that NMHC-A mRNA is predominantly expressed in the epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes and branchial arches. Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentiation proceeds. DAPI nuclear staining demonstrated that NMHC-B mRNA is colocalized with the nuclei or perinuclear area. In animal cap experiments, treatment with activin A or ectopic expression of Xbra and an activated form of Xlim1 markedly up-regulates NMHC-B as well as muscle actin mRNAs and slightly down-regulates NMHC-A mRNA, consistent with NMHC-B expression in the somitic muscle and NMHC-A expression in the epidermis.

Nonmuscle myosin II-B is required for normal development of the mouse heart.

We used targeted gene disruption in mice to ablate nonmuscle myosin heavy chain B (NMHC-B), one of the two isoforms of nonmuscle myosin II present in all vertebrate cells. Approximately 65% of the NMHC-B-/- embryos died prior to birth, and those that were born suffered from congestive heart failure and died during the first day. No abnormalities were detected in NMHC-B+/- mice. The absence of NMHC-B resulted in a significant increase in the transverse diameters of the cardiac myocytes from 7.8 +/- 1.8 micron (right ventricle) and 7.8 +/- 1.3 micron (left ventricle) in NMHC-B+/+ and B+/- mice to 14.7 +/- 1.1 micron and 13.8 +/- 2.3 micron, respectively, in NMHC-B-/- mice (in both cases, P < 0.001). The increase in size of the cardiac myocytes was seen as early as embryonic day 12.5 (4.5 +/- 0.2 micron for NMHC-B+/+ and B+/- vs. 7. 2 +/- 0.6 micron for NMHC-B-/- mice (P < 0.01)). Six of seven NMHC-B-/- newborn mice analyzed by serial sectioning also showed structural cardiac defects, including a ventricular septal defect, an aortic root that either straddled the defect or originated from the right ventricle, and muscular obstruction to right ventricular outflow. Some of the hearts of NMHC-B-/- mice showed evidence for up-regulation of NMHC-A protein. These studies suggest that nonmuscle myosin II-B is required for normal cardiac myocyte development and that its absence results in structural defects resembling, in part, two common human congenital heart diseases, tetralogy of Fallot and double outlet right ventricle.

Xenopus nonmuscle myosin heavy chain isoforms have different subcellular localizations and enzymatic activities.

There are two isoforms of the vertebrate nonmuscle myosin heavy chain, MHC-A and MHC-B, that are encoded by two separate genes. We compared the enzymatic activities as well as the subcellular localizations of these isoforms in Xenopus cells. MHC-A and MHC-B were purified from cells by immunoprecipitation with isoform-specific peptide antibodies followed by elution with their cognate peptides. Using an in vitro motility assay, we found that the velocity of movement of actin filaments by MHC-A was 3.3-fold faster than that by MHC-B. Likewise, the Vmax of the actin-activated Mg(2+)-ATPase activity of MHC-A was 2.6-fold greater than that of MHC-B. Immunofluorescence microscopy demonstrated distinct localizations for MHC-A and MHC-B. In interphase cells, MHC-B was present in the cell cortex and diffusely arranged in the cytoplasm. In highly polarized, rapidly migrating interphase cells, the lamellipodium was dramatically enriched for MHC-B suggesting a possible involvement of MHC-B based contractions in leading edge extension and/or retraction. In contrast, MHC-A was absent from the cell periphery and was arranged in a fibrillar staining pattern in the cytoplasm. The two myosin heavy chain isoforms also had distinct localizations throughout mitosis. During prophase, the MHC-B redistributed to the nuclear membrane, and then resumed its interphase localization by metaphase. MHC-A, while diffuse within the cytoplasm at all stages of mitosis, also localized to the mitotic spindle in two different cultured cell lines as well as in Xenopus blastomeres. During telophase both isoforms colocalized to the contractile ring. The different subcellular localizations of MHC-A and MHC-B, together with the data demonstrating that these myosins have markedly different enzymatic activities, strongly suggests that they have different functions.

Identification of the chimeric protein product of the CBFB-MYH11 fusion gene in inv(16) leukemia cells.

An expressed gene formed by fusion between the CBFB transcription factor gene and the smooth muscle myosin heavy chain gene MYH11 is consistently detected by reverse transcription polymerase chain reaction (RT-PCR) in patients who have acute myeloid leukemia (AML) subtype M4Eo with an inversion of chromosome 16. We have previously shown that a CBFB-MYH11 cDNA construct can produce a chimeric protein and transform NIH 3T3 cells. However, the presence of the chimeric protein in patient cells has not been demonstrated previously. Here, we show that such chimeric proteins can be identified in vivo, primarily in the nuclei of the leukemic cells, by use of antibodies against the C-terminus of the smooth muscle myosin heavy chain and the fusion junction peptide. A very high molecular weight protein/DNA complex is generated when nuclear extracts from patient cells are used in electrophoretic mobility shift assays, as seen in NIH 3T3 cells transfected with the CBFB-MYH11 cDNA. Immunofluorescence staining shows that the proteins are organized in vivo into novel structures within cell nuclei. One isoform of the transcript of the CBFB-MYH11 fusion gene, containing the MHC204 C-terminus, was the predominant from in all five cases studied.

Baculovirus expression of chicken nonmuscle heavy meromyosin II-B. Characterization of alternatively spliced isoforms.

We have expressed two truncated isoforms of chicken nonmuscle myosin II-B using the baculovirus expression system. One of the expressed heavy meromyosins (HMMexp) consists of two 150-kDa myosin heavy chains (MHCs), comprising amino acids 1-1231 as well as two pairs of 20-kDa and 17-kDa myosin light chains (MLCs) in a 1:1:1 molar ratio. The second HMMexp was identical except that it contained an insert of 10 amino acids (PESPKPVKHQ) at the 25-50-kDa domain boundary in the subfragment-1 region of the MHC. These 10 amino acids include a consensus sequence (SPK) for proline-directed kinases. Expressed HMMs were soluble at low ionic strength and bound to rabbit skeletal muscle actin in an ATP-dependent manner. These properties afforded a rapid purification of milligram quantities of expressed protein. Both isoforms were capable of moving actin filaments in an in vitro motility assay and manifested a greater than 20-fold activation of actin-activated MgATPase activity following phosphorylation of the 20-kDa MLC. HMMexp with the 10-amino acid insert was phosphorylated by Cdc2, Cdk5, and mitogen-activated protein kinase in vitro to 0.3-0.4 mol of PO4/mol of MHC. The site phosphorylated in the MHC was identified as the serine residue present in the 10-amino acid insert and its presence was confirmed in bovine brain MHCs. Characterization of the baculovirus expressed noninserted and inserted MHC isoforms with respect to actin-activated MgATPase activity and ability to translocate actin filaments in an in vitro motility assay produced the following average values following MLC phosphorylation: noninserted HMMexp, Vmax = 0.28 s-1, Km = 12.7 microM; translocation rate = 0.077 micron/s; inserted HMMexp, Vmax = 0.37 s-1, Km = 15.1 microM; translocation rate = 0.092 micron/s.

Cloning of the cDNA encoding rat myosin heavy chain-A and evidence for the absence of myosin heavy chain-B in cultured rat mast (RBL-2H3) cells.

The complete amino acid sequence (1961 amino acids) of a vertebrate cellular myosin heavy chain-A was deduced from cDNA clones of a secretory rat mast cell line, the RBL-2H3 cell. The rat, human and chicken cellular myosin heavy chain-A exhibited high similarity in domains that allow binding of ATP and actin. The amino acid sequence of non-muscle myosin heavy chain-A from rat was 96% identical to that in human and 92% identical to that in chicken. Northern blot analysis of mRNA indicated the presence of single message of 7.4 kilobases. Northern blot, reverse-transcriptase polymerase chain reaction, and Western blot with isoform-specific antibodies indicated that RBL-2H3 cells expressed exclusively myosin heavy chain-A. Unlike rat PC12 cells, as well as a wide variety of other cultured cells and tissues, myosin heavy chain-B mRNA and protein were not detectable in RBL-2H3 cells. Because RBL-2H3 cells can be stimulated to release secretory granules as well as newly generated arachidonic acid and cytokines but lack myosin heavy chain-B, this cell line may provide a unique model to study the role of myosin heavy chain-A in cellular responses to antigen and other stimulants.

Localization of myosin II A and B isoforms in cultured neurons.

Tension generated by growth cones regulates both the rate and the direction of neurite growth. The most likely effectors of tension generation are actin and myosins. We are investigating the role of conventional myosin in growth cone advance. In this paper we report the localization of the two most prominent isoforms of brain myosin II in growth cones, neurites and cell bodies of rat superior cervical ganglion neurons. Affinity purified polyclonal antibodies were prepared against unique peptide sequences from human and rat A and B isoforms of myosin heavy chain. Although each of these antibodies brightly stained nonneuronal cells, antibodies to myosin heavy chain B stained neurons with greater intensity than antibodies to myosin heavy chain A. In growth cones, myosin heavy chain B was most concentrated in the margin bordering the thickened, organelle-rich central region and the thin, actin-rich peripheral region. The staining colocalized with actin bundles proximal and distal to the marginal zone, though the staining was more prominent proximally. The trailing edge of growth cones and the distal portion of the neurite often had a rimmed appearance, but more proximal regions of neurites had cytoplasmic labelling. Localizing MHC-B in growth cones previously monitored during advance (using differential interference contrast microscopy) revealed a positive correlation with edges at which retraction had just occurred and a negative correlation with lamellipodia that had recently undergone protrusion. Cell bodies were brightly labelled for myosin heavy chain B. Myosin heavy chain A staining was dimmer and its colocalization with filamentous actin bundles in growth cones was less striking than that of myosin heavy chain B. Growth cones stained for both myosin heavy chain A and B revealed that the two antigens overlapped frequently, but not exclusively, and that myosin heavy chain A lacked the elevation in the marginal zone that was characteristic of myosin heavy chain B. The pattern of staining we observed is consistent with a prominent role for myosin heavy chain B in either generating tension between widely separated areas of the growth cone, or bundling of actin filaments, which would enable other motors to effect this tension. These data support the notion that conventional myosin is important in growth cone advance and turning.

Cloning of the cDNA encoding human nonmuscle myosin heavy chain-B and analysis of human tissues with isoform-specific antibodies.

Previously, we reported the sequence of cDNA clones encoding amino acids 63 through 723 of the human nonmuscle myosin heavy chain-B isoform. In this paper, we present the derived sequence of the remaining 1303 amino acids along with 5' and 3' untranslated sequences. We made use of the differences between the derived nonmuscle myosin heavy chain-A and -B amino acid sequences to raise isoform-specific antibodies. Immunoblot analysis reveals a differential expression of both myosin heavy chain isoforms in a variety of human adult and foetal tissues and cells. When extracts of human adult aorta were subjected to gel electrophoresis, two distinct Coomassie Blue-stained bands and a fused band were seen migrating at approximately 200 kDa. These bands can be detected with four different specific antibodies recognizing the two different smooth muscle myosin heavy chain isoforms (204 kDa and 200 kDa) and the two different nonmuscle myosin heavy chain isoforms (A and B). Using immunohistochemistry, we confirmed the presence of the four different isoforms in adult and foetal aortas.

Neuronal cell expression of inserted isoforms of vertebrate nonmuscle myosin heavy chain II-B.

Previous work has demonstrated that unique isoforms of nonmuscle myosin heavy chain II-B (MHC-B) are expressed in chicken and human neuronal cells (Takahashi, M., Kawamoto, S., and Adelstein, R. S. (1992) J. Biol. Chem. 267, 17864-17871). These isoforms, which appear to be generated by alternative splicing of pre-mRNA, differ from the MHC-B isoform present in a large number of nonmuscle cells in that they contain inserted cassettes of amino acids near the ATP binding region and/or near the actin binding region. The insert near the ATP binding region begins after amino acid 211 and consists of either 10 or 16 amino acids. The insert near the actin binding region begins after amino acid 621 and consists of 21 amino acids. Using a variety of techniques, we have studied the distribution and expression of the inserted MHC-B isoforms. In the developing chicken brain, mRNA encoding the 10-amino acid insert gradually increases after embryonic day 4, peaks in the 10-14-day embryo, and then declines. In contrast, the mRNA encoding the 21-amino acid insert appears just before birth and is abundantly expressed in the adult chicken cerebellum. There is a marked species difference between the distribution of the inserted isoforms in adult tissues. The mRNA encoding MHC-B containing the 10-amino acid insert near the ATP binding region is expressed at low levels in the adult chicken brain, but makes up most of the MHC-B mRNA expressed in the human cerebrum and approximately 90% of MHC-B in the human retina. It is also expressed in neuronal cell lines. The mRNA encoding MHC-B containing the 21-amino acid insert is abundantly expressed in the chicken cerebellum and human cerebrum, but is absent from the retina and cell lines. Employing human retinoblastoma (Y-79) and neuroblastoma (SK-N-SH) cell lines, an increase in expression of mRNA encoding the 10-amino acid inserted isoform was seen following treatment by a number of agonists or by serum deprivation. In each case, expression of the inserted MHC-B isoform correlated with cell differentiation (neuronal phenotype) and inhibition of cell division. Using a rat pheochromocytoma cell line (PC12), we found that prior to treatment with nerve growth factor (NGF), there was no evidence for either inserted isoform, although noninserted MHC-B was present. NGF treatment resulted in the appearance of mRNA encoding MHC-B containing the 10-amino acid insert, concomitant with neurite outgrowth.(ABSTRACT TRUNCATED AT 250 WORDS)

The leukemic core binding factor beta-smooth muscle myosin heavy chain (CBF beta-SMMHC) chimeric protein requires both CBF beta and myosin heavy chain domains for transformation of NIH 3T3 cells.

An inversion of chromosome 16 associated with the M4Eo subtype of acute myeloid leukemia produces a chimeric protein fusing the beta subunit of the transcription factor core binding factor (CBF beta) to the tail region of smooth muscle myosin heavy chain (SMMHC). We investigated the oncogenic properties of this CBF beta-SMMHC chimeric protein using a 3T3 transformation assay. NIH 3T3 cells expressing CBF beta-SMMHC acquired a transformed phenotype, as indicated by their ability to form foci, grow in soft agarose, and form tumors in nude mice. Cells expressing normal CBF beta or the SMMHC tail domain did not become transformed. Electrophoretic mobility-shift assays showed that extracts from cells transformed by CBF beta-SMMHC no longer formed the normal CBF/DNA complex but instead formed a much larger complex that did not migrate into the gel. Analysis of CBF beta-SMMHC deletion mutants demonstrated that the chimeric protein was transforming only if two domains were both present: (i) CBF beta sequences necessary for association with the CBF alpha subunit, and (ii) SMMHC sequences important for the formation of multimeric filaments. These results are direct evidence that CBF beta-SMMHC can function as an oncoprotein.