Distribution of SRPK1 in human brain.

Extensive alternative splicing is observed in the mammalian nervous system providing for protein diversity and specificity to accomplish the complex neuronal functions. Mechanisms underlying neuron specific splicing are not yet well understood. Among the factors regulating splicing of major importance are serine/arginine protein kinases (SRPKs) that phosphorylate SR splicing factors. SRPK1 is known to be expressed in the mammalian central nervous system. The present immunohistochemical study reveals a region- and neuron-specific localization of SRPK1 in human brain. The potential involvement of the kinase in the regulation of alternative splicing of various neuronal proteins is discussed.

Histones H3/H4 form a tight complex with the inner nuclear membrane protein LBR and heterochromatin protein 1.

We have recently shown that heterochromatin protein 1 (HP1) interacts with the nuclear envelope in an acetylation-dependent manner. Using purified components and in vitro assays, we now demonstrate that HP1 forms a quaternary complex with the inner nuclear membrane protein LBR and a sub-set of core histones. This complex involves histone H3/H4 oligomers, which mediate binding of LBR to HP1 and cross-link these two proteins that do not interact directly with each other. Consistent with previous observations, HP1 and LBR binding to core histones is strongly inhibited when H3/H4 are modified by recombinant CREB-binding protein, revealing a new mechanism for anchoring domains of under-acetylated chromatin to the inner nuclear membrane.

Cloning and characterization of an alternatively spliced form of SR protein kinase 1 that interacts specifically with scaffold attachment factor-B.

Serine/arginine protein kinases have been conserved throughout evolution and are thought to play important roles in the regulation of mRNA processing, nuclear import, germline development, polyamine transport, and ion homeostasis. Human SRPK1, which was first identified as a kinase specific for the SR family of splicing factors, is located on chromosome 6p21.2-p21.3. We report here the cloning and characterization of SRPK1a, which is encoded by an alternatively processed transcript derived from the SRPK1 gene. SRPK1a contains an insertion of 171 amino acids at its NH(2)-terminal domain and is similar to SRPK1 in substrate specificity and subcellular localization. Moreover, both isoforms can induce alternative splicing of human tau exon 10 in transfected cells. Using the yeast two-hybrid assay, we found that the extended NH(2)-terminal domain of SRPK1a interacts with Scaffold Attachment Factor-B, a nuclear scaffold-associated protein. Confirmation of this interaction was provided by in vitro binding assays, as well as by co-immunoprecipitation from 293T cells doubly transfected with SRPK1a and SAF-B. Our studies suggest that different SRPK family members are uniquely regulated and targeted and thus the multiple SRPK kinases present in higher eukaryotes may perform specialized and differentiable functions.

Regulation of alternative splicing of human tau exon 10 by phosphorylation of splicing factors.

Tau is a microtubule-associated protein whose transcript undergoes regulated splicing in the mammalian nervous system. Exon 10 of the gene is an alternatively spliced cassette that is adult-specific and encodes a microtubule-binding domain. Mutations increasing the inclusion of exon 10 result in the production of tau protein which predominantly contains four microtubule-binding repeats and were shown to cause frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Here we show that exon 10 usage is regulated by CDC2-like kinases CLK1, 2, 3, and 4 that phosphorylate serine-arginine-rich proteins, which in turn regulate pre-mRNA splicing. Cotransfection experiments suggest that CLKs achieve this effect by releasing specific proteins from nuclear storage sites. Our results show that changing pre-mRNA-processing pathways through phosphorylation could be a new therapeutic concept for tauopathies.

Purification and characterization of a dimer form of the cAMP-dependent protein kinase from mouse liver cytosol.

A protein kinase that phosphorylates histones and polysomal proteins was partially purified from mouse liver cytosol. The active enzyme has a molecular mass of 100 kDa and a phosphorylatable subunit of 54 kDa. Biochemical as well as immunological data suggest that the enzyme is a heterodimer composed of the catalytic subunit of cyclic AMP-dependent protein kinase and the RII regulatory subunit. This RC form does not seem to dissociate upon activation with 3', 5' cyclic AMP and exhibits identical specificity as the classical cAMP-dependent protein kinase (2.7.1.37). The enzyme is affected by the 3', 5' cyclic phosphates of adenosine mainly, but also of guanosine, uridine and cytidine in a substrate-dependent manner. Cyclic nucleotides slightly stimulate phosphate incorporation into histones, while phosphorylation of polysomal proteins in intact polysomes is dramatically increased. The substrate- specific stimulatory effects of 3', 5' cyclic nucleotides are due to repression of the inhibition exerted upon the reaction, by negatively charged macromolecules such as RNA, DNA and to a lesser extent heparin.

SR protein-specific kinase 1 is highly expressed in testis and phosphorylates protamine 1.

Arginine/serine protein kinases constitute a novel class of enzymes that can modify arginine/serine (RS) dipeptide motifs. SR splicing factors that are essential for pre-mRNA splicing are among the best characterized proteins that contain RS domains. TwoSRprotein-specifickinases, SRPK1 and SRPK2, have been considered as highly specific for the phosphorylation of these proteins, thereby contributing to splicing regulation. However, despite the fact that SR proteins are more or less conserved among metazoa and have a rather ubiquitous tissue distribution we now demonstrate that SRPK1 is predominantly expressed in testis. In situ expression analysis on transverse sections of adult mouse testis shows that SRPK1 mRNA is abundant in all germinal cells but not in mature spermatozoa. RS kinase activity was found primarily in the cytosol and only minimal activity was detected in the nucleus. In a search for testis-specific substrates of SRPK1 we found that the enzyme phosphorylates human protamine 1 as well as a cytoplasmic pool of SR proteins present in the testis. Protamine 1 belongs to a family of small basic arginine-rich proteins that replace histones during the development of mature spermatozoa. The result of this progressive replacement is the formation of a highly compact chromatin structure devoid of any transcriptional activity. These findings indicate that SRPK1 may have a role not only in pre-mRNA splicing, but also in the condensation of sperm chromatin.

SRPK1 and LBR protein kinases show identical substrate specificities.

Arginine/serine protein kinases constitute a novel class of enzymes that can modify arginine/serine (RS) dipeptide motifs. SR splicing factors that are essential for pre-mRNA splicing and the lamin B receptor (LBR), an integral protein of the inner nuclear membrane, are among the best characterized proteins that contain RS domains. Two SR Protein-specific Kinases, SRPK1 and SRPK2, have been shown to phosphorylate specifically the RS motifs of the SR family of splicing factors and play an important role in regulating both the spliceosome assembly and their intranuclear distribution, whereas an LBR-associated kinase, that specifically phosphorylates a stretch of RS repeats located at the NH2-terminal region of LBR, has been recently purified and characterized from turkey erythrocyte nuclear envelopes. Using synthetic peptides representing different regions of LBR and recombinant proteins produced in bacteria we now demonstrate that SRPK1 modifies LBR with similar kinetics and on the same sites as the LBR kinase, that are also phosphorylated in vivo. These data provide significant evidence for a new role of SRPK1 in addition to that of pre-mRNA splicing.

Mitotic phosphorylation of the lamin B receptor by a serine/arginine kinase and p34(cdc2).

The lamin B receptor (LBR) is an integral protein of the inner nuclear membrane that is modified at interphase by a nuclear envelope-bound protein kinase. This enzyme (RS kinase) specifically phosphorylates arginine-serine dipeptide motifs located at the NH2-terminal domain of LBR and regulates its interactions with other nuclear envelope proteins. To compare the phosphorylation state of LBR during interphase and mitosis, we performed phosphopeptide mapping of in vitro and in vivo 32P-labeled LBR and analyzed a series of recombinant proteins and synthetic peptides. Our results show that LBR undergoes two types of mitotic phosphorylation mediated by the RS and the p34(cdc2) protein kinases, respectively. The RS kinase modifies similar sites at interphase and mitosis (i.e. Ser76, Ser78, Ser80, Ser82, Ser84), whereas p34(cdc2) mainly phosphorylates Ser71. These findings clarify the phosphorylation state of LBR during the cell cycle and provide new information for understanding the mechanisms responsible for nuclear envelope assembly and disassembly.

A nuclear envelope-associated kinase phosphorylates arginine-serine motifs and modulates interactions between the lamin B receptor and other nuclear proteins.

Previous studies have identified a subassembly of nuclear envelope proteins, termed "the LBR complex." This complex includes the lamin B receptor protein (LBR or p58), a kinase which phosphorylates LBR in a constitutive fashion (LBR kinase), the nuclear lamins A and B, an 18-kDa polypeptide (p18), and a 34-kDa protein (p34/p32). The latter polypeptide has been shown to interact with the HIV-1 proteins Rev and Tat and with the splicing factor 2 (SF2). Using recombinant proteins produced in bacteria and synthetic peptides representing different regions of LBR, we now show that the LBR kinase modifies specifically arginine-serine (RS) dipeptide motifs located at the nucleoplasmic, NH2-terminal domain of LBR and in members of the SR family of splicing factors. Furthermore, we show that the NH2-terminal domain of LBR binds to p34/p32, whereas a mutated domain lacking the RS region does not. Phosphorylation of LBR by the RS kinase completely abolishes binding of p34/p32, suggesting that this enzyme regulates interactions among the components of the LBR complex.

Post-translational processing of Schizosaccharomyces pombe YPT5 protein. In vitro and in vivo analysis of processing mutants.

SpYPT5p is a member of the rab/YPT small GTP-binding protein family, which is believed to be involved in the regulation of intracellular trafficking. The protein sequence terminates with a CXC motif, and in our previous report (Newman, C. M. H., Giannakouros, T., Hancock, J. F., Fawell, E. H., Armstrong, J., and Magee, A. I. (1992) J. Biol. Chem. 267, 11329-11336) we have shown that SpYPT5p is prenylated both in vivo and in vitro, where geranylgeranylation was confirmed, and carboxyl-methylated. In order to dissect the role of prenylation of each cysteine, we have generated C-terminal mutants where either one or both cysteine(s) were replaced by serine and expressed them in vitro in reticulocyte lysates and in vivo in transfected COS cells. Our results suggest that both cysteines of the CXC motif are prenylated but that the rate of prenylation of the two cysteines is different. The upstream cysteine was found to be preferentially prenylated in reticulocyte lysates unless cytosol from COS cells was added. A separate activity could therefore be required for prenylation of the second cysteine, or the presence of an additional factor is needed to allow accumulation of doubly prenylated SpYPT5p. However, the modification of the upstream cysteine is not a prerequisite for the prenylation of the other. Furthermore, gene replacement in Schizosaccharomyces pombe revealed that each cysteine of the CXC motif can individually support function. Carboxyl methylation occurred only on protein which had been prenylated on the C-terminal cysteine and was required for efficient membrane binding in vitro.

Processing of the small GTP-binding protein SpYPT1p in Schizosaccharomyces pombe and in mammalian COS cells.

SpYPT1p belongs to a family of ras-like GTP-binding proteins which is believed to be involved in the regulation of intracellular vesicular trafficking. We have analyzed the processing of this protein in its natural environment, the fission yeast Schizosaccharomyces pombe, and when expressed in transfected mammalian COS cells. In COS cells SpYPT1p exists in two forms: a cytosolic 24 kDa protein which represents the unprocessed precursor form and a prenylated 23.5 kDa protein which is equally distributed between membranes and cytosol. In contrast, in S.pombe we have been unable to detect any prenylation of the protein, despite the presence of a potential C-terminal CysCys prenylation site. In addition a 23.5 kDa form was localized exclusively in the cytosol and a third membrane-bound 23 kDa form was also detected. Pulse-chase experiments revealed that in S.pombe SpYPT1p is co-translationally or immediately after translation converted to the 23.5 kDa form which is then rapidly processed to the 23 kDa membrane bound form. We have been unable to detect any significant soluble pool of the protein.

Post-translational processing of Schizosaccharomyces pombe YPT proteins.

ras proteins are post-translationally processed at their carboxyl-terminal CAAX motif by a triplet of modifications: prenylation of C with farnesyl, proteolytic trimming of AAX, and carboxyl-methylation. These modifications co-operate with palmitoylation of nearby sites or a polybasic region to target plasma membrane localization. The related YPT/rab proteins in contrast are localized to compartments of the endo-membrane system and may be involved in directing membrane traffic. These proteins end in XCC or CXC motifs. We have analyzed the processing of members of this subfamily form the fission yeast Schizosaccharomyces pombe. We find using in vitro translation in reticulocyte lysates that YPT1, -3, and -5 are prenylated with geranylgeranyl and that they incorporate label from [3H]mevalonic acid when expressed in transfected COS cells in vivo. Furthermore, prenylation was necessary for membrane binding in vivo. The CXC protein YPT5, but neither of the two XCC proteins YPT1 and YPT3, was carboxyl-methylated in S. pombe and in COS cells in vivo. However, YPT5 was not carboxyl-methylated in vitro in lysates which were able to methylate ras protein. YPT3 was detectably palmitoylated when expressed in COS cells, though at a much lower level than ras.

Protein prenylation in Schizosaccharomyces pombe.

S. pombe is shown to be a powerful system for studies concerning attachment of polyisoprenoid moieties to proteins, due to its ability to take up exogenous mevalonic acid efficiently. The fission yeast can take up about 5% of the exogenously added mevalonic acid and incorporate approximately 10% of this into protein. By contrast, the uptake obtained with the budding yeast S. cerevisiae is less than 0.5%. HPLC analysis of total S. pombe protein-bound isoprenoids revealed that approximately 55% of the counts co-migrated with the geranylgeraniol standard, while approximately 45% of the counts co-migrated with farnesol. We could not detect any effects of mevinolin or other HMG-CoA reductase inhibitors in S. pombe.

Concentration-dependent effects of natural polyamines on peptide chain initiation and elongation in a cell-free system of protein synthesis.

Spermidine and spermine at submillimolar concentrations stimulate the rate of incorporation of amino acid into protein in a cell-free system, directed either by endogenous or exogenous mRNA (TMV, globin). The stimulatory effects of these polyamines are exerted at both the stages of initiation and elogation and are more pronounced in the case of TMV or globin mRNA, amounting to approximately 2.3-fold stimulation over the polyamine-free system. The number of polysomes and the polysome-associated radioactivity increase approximately 2-fold in the presence of spermine. Synthesis of large polypeptides is a characteristic feature of the stimulatory event. However, elevated concentrations of spermidine and spermine strongly inhibit amino acid incorporation into protein. Inhibition is manifest at the stage of peptide elongation. In the case of endogenous mRNA the addition of an excess of polyamines results in a non uniform inhibition of amino acid incorporation. A most interesting finding is that, with increasing concentrations of polyamines, the intensity of four bands with Mr values of 63000, 44000, 15500 and 12500 respectively, increases or leastwise remains constant while others fade, indicating differential translation of proteins in the presence of polyamines.

Inhibition of protein synthesis by acetyl-coenzyme A in a cell-free system: possible involvement of protein acetylation in the regulation of translation.

Acetyl-coenzyme A (CoASAc) inhibits the rate of incorporation of amino acid into protein in a cell-free system of mouse liver. The effect is more pronounced when exogenous mRNA (tobacco mosaic virus or globin mRNA) rather than endogenous messages are used. Micromolar concentrations of the cofactor block initiation, while millimolar concentrations cause a more general inhibition of the translation process, that affects, in addition, the elongation step. Inclusion of [1-14C]acetyl-CoA in a protein synthesis reaction mixture results in a very rapid and selective labelling of a protein of 200 kd of the 'pH 5' fraction. The possible involvement of the acetylating event in the regulation of protein synthesis is discussed.

A high-yield cell-free system of protein synthesis of mouse liver.

1. A fractionated cell-free system of protein synthesis has been developed from mouse liver. It is composed of polysomes, "pH 5" fraction, Mg2+, K+, ATP and a ATP generating system. 2. It operates optimally at 30-37 degrees C, in the presence of 4 mM MgCl2 and 90 mM KCl. 3. Spermine is highly inhibitory, while spermidine shows a bimodal action, in that submillimolar concentrations stimulate, while millimolar concentrations inhibit protein synthesis. 4. Both spermine and spermidine show an interesting selectivity, in that, even though they inhibit incorporation of amino acids into most proteins, they stimulate incorporation into a few proteins. 5. The system can be rendered mRNA-dependent, either by preincubation or by treatment with micrococcal nuclease. In both cases globin mRNA as well as TMV RNA are faithfully translated. 6. Compared to other published mammalian fractionated cell-free systems, the mouse liver system is more efficient by approximately one order of magnitude, since the rate of incorporation of leucine per min is 30 pmol/mg protein or 435 pmol/mg RNA or 1 mol/mol ribosomes.