Recruitment of adenomatous polyposis coli and beta-catenin to axin-puncta.

The adenomatous polyposis coli (APC) tumour suppressor is a multifunctional protein involved in the regulation of Wnt signalling and cytoskeletal dynamics. Little is known about how APC controls these disparate functions. In this study, we have used APC- and axin-fluorescent fusion proteins to examine the interactions between these proteins and show that the functionally distinct populations of APC are also spatially separate. Axin-RFP forms cytoplasmic punctate structures, similar to endogenous axin puncta. Axin-RFP recruits beta-catenin destruction complex proteins, including APC, beta-catenin, glycogen synthase kinase-3-beta (GSK3-beta) and casein kinase-1-alpha (CK1-alpha). Recruitment into axin-RFP puncta sequesters APC from clusters at cell extensions and this prevents its microtubule-associated functions. The interaction between APC-GFP and axin-RFP within the cytoplasmic puncta is direct and dramatically alters the dynamic properties of APC-GFP. However, recruitment of APC to axin puncta is not absolutely required for beta-catenin degradation. Instead, formation of axin puncta, mediated by the DIX domain, is required for beta-catenin degradation. An axinDeltaDIX mutant did not form puncta, but still mediated recruitment of destruction complex proteins and phosphorylation of beta-catenin. We conclude that there are distinct pools of APC and that the formation of axin puncta, rather than the axin/APC complex, is essential for beta-catenin destruction.

Frizzled-7 dictates three-dimensional organization of colorectal cancer cell carcinoids.

Progression of colorectal cancer (CRC) involves spatial and temporal occurrences of epithelial-mesenchymal transition (EMT), whereby tumour cells acquire a more invasive and metastatic phenotype. Subsequently, the disseminated mesenchymal tumour cells must undergo a reverse transition (mesenchymal-epithelial transition, MET) at the site of metastases, as most metastases recapitulate the pathology of their corresponding primary tumours. Importantly, initiation of tumour growth at the secondary site is the rate-limiting step in metastasis. However, investigation of this dynamic reversible EMT and MET that underpins CRC morphogenesis has been hindered by a lack of suitable in vitro models. To this end, we have established a unique in vitro model of CRC morphogenesis, which we term LIM1863-Mph (morphogenetic). LIM1863-Mph cells spontaneously undergo cyclic transitions between two-dimensional monolayer (migratory, mesenchymal) and three-dimensional sphere (carcinoid, epithelial) states. Using RNAi, we demonstrate that FZD7 is necessary for MET of the monolayer cells as loss of FZD7 results in the persistence of a mesenchymal state (increased SNAI2/decreased E-cadherin). Moreover, FZD7 is also required for migration of the LIM1863-Mph monolayer cells. During development, FZD7 orchestrates either migratory or epithelialization events depending on the context. Our findings strongly implicate similar functional diversity for FZD7 during CRC morphogenesis.

Structural and biosensor analyses of a synthetic biotinylated peptide probe for the isolation of adenomatous polyposis coli tumor suppressor protein complexes.

Large numbers of colon tumors stem from mutations in the gene coding for the production of the adenomatous polyposis coli (APC) tumor suppressor protein. This protein contains a coiled-coil N-terminal domain that is known to be responsible for homodimerization. Previous work by others has led to the design of a specific 54-residue anti-APC peptide (anti-APCp1) that dimerizes preferentially with this domain. We have undertaken the chemical synthesis of a modified form of this peptide (anti-APCp2) that bears a biotin moiety at its N-terminus for use in subsequent ligand-binding analysis studies. The peptide was subjected to comprehensive chemical characterization to confirm its purity. Secondary structural analysis by circular dichroism spectroscopy and Fourier transform infrared spectroscopy indicated that the peptide could assume a wide range of potential conformations, depending upon the precise microenvironment. Significantly, a stable alpha-helical structure was generated when the solvent conditions supported intramolecular salt-bridge formation along the helix barrel. The biotinylated anti-APCp2 was immobilized onto a streptavidin sensor surface, in a specific orientation leaving all amino acids available to form a coiled structure. In one experiment, injection of colonic cell lysate extracts (LIM1215) onto a size-exclusion column resulted in the isolation of a high molecular mass protein peak (> 600 kDa) that reacted specifically with the immobilized anti-APCp2 on the biosensor surface. In another experiment, a high molecular mass protein (M(r) > 250 kDa on SDS-PAGE) could be specifically immunoprecipitated from this peak using either the anti-APCp2 peptide or an anti-APC polyclonal antibody. This demonstrates the specific interaction between the anti-APCp2 peptide and native APC and highlights the potential use of the former peptide in a multidimensional micropreparative chromatographic/biosensor/proteomic protocol for the purification of APC alone and APC complexed with different biopolymers in various cell lines, and stages of tumor development.

Targeted protein kinase A and PP-2B regulate insulin secretion through reversible phosphorylation.

Protein kinases and phosphatases play key roles in integrating signals from various insulin secretagogues. In this study, we show that the activities of the cAMP-dependent protein kinase (PKA) and the calcium/calmodulin-dependent phosphatase, PP-2B are coordinated resulting in the regulation of insulin secretion. Transient inhibition of PP-2B, using the immunosuppressant FK506, increased forskolin stimulated insulin secretion by 2.5-fold +/- 0.3 (n = 6) in rat islets and RINm5F cells. Surprisingly, forskolin treatment resulted in the dephosphorylation of the vesicle-associated protein synapsin 1 and increased PP-2B activity by 2.98 +/- 0.97-fold (n = 4). One potential explanation for the observed coordination of PKA and PP-2B activity is their colocalization through a mutual anchoring protein, AKAP79/150. Accordingly, RINm5F cells expressing AKAP79 exhibited decreased insulin secretion, reduced PP-2B activity and were insensitive to FK506. This suggests that AKAP targeting of PKA and PP-2B maintains a signal transduction complex that may regulate reversible phosphorylation events involved in insulin secretion.

The use of coiled-coil interactions for the analysis and micropreparative isolation of adenomatous polyposis coli protein complexes.

The coiled coil is a common structural motif found both as the dominant structure in fibrous proteins and as an oligomerization domain in a variety of cytoskeletal and extracellular matrix proteins. Coiled-coils typically consist of two to four helices that are supercoiled around one another in either parallel or antiparallel orientations. In the past few years our knowledge of the structure and specificity of coiled coil interactions has increased, allowing the de novo design and preparation of coiled-coils with well-defined structure and specificity. Indeed, the design and synthesis of a peptide that binds specifically to a single coiled-coil-containing protein, adenomatous polyposis coli (APC) has been reported. We have optimized solid-phase synthesis techniques to produce a modified form of the anti-APC peptide that contains a biotin moiety specifically placed so as to allow selective orientation onto the surface of a biosensor or affinity support. These peptide surfaces have been used to both monitor and purify APC and APC complexes from cellular extracts.

Mechanism of A-kinase-anchoring protein 79 (AKAP79) and protein kinase C interaction.

The A-kinase-anchoring protein AKAP79 co-ordinates the location of cAMP-dependent protein kinase, phosphatase 2B (PP2B/calcineurin) and protein kinase C (PKC) at postsynaptic sites in neurons. In this report we focus on the mechanism of interaction between AKAP79 and PKC. We show that neither lipid activators nor kinase activation are required for association with AKAP79. The anchoring protein binds and inhibits the conserved catalytic core of PKCbetaII. AKAP79 also associates with conventional, novel and atypical isoforms of PKC in vitro and in vivo, and immunofluorescence staining of rat hippocampal neurons demonstrates that the murine anchoring-protein homologue AKAP150 is co-distributed with PKCalpha/beta, PKCepsilon or PKCiota. Binding of the AKAP79(31-52) peptide, which inhibits kinase activity, exposes the pseudosubstrate domain of PKCbetaII, allowing endoproteinase Arg-C proteolysis in the absence of kinase activators. Reciprocal experiments have identified two arginine residues at positions 39 and 40 that are essential for AKAP79(31-52) peptide inhibition of PKCbetaII. Likewise, the same mutations in the full-length anchoring protein reduced inhibition of PKCbetaII. Thus AKAP79 associates with multiple PKC isoforms through a mechanism involving protein-protein interactions at the catalytic core where binding of the anchoring protein inhibits kinase activity through displacement of the pseudosubstrate.

Carboxyl-terminal phosphorylation regulates the function and subcellular localization of protein kinase C betaII.

Protein kinase C is processed by three phosphorylation events before it is competent to respond to second messengers. Specifically, the enzyme is first phosphorylated at the activation loop by another kinase, followed by two ordered autophosphorylations at the carboxyl terminus (Keranen, L. M., Dutil, E. M., and Newton, A. C. (1995) Curr. Biol. 5, 1394-1403). This study examines the role of negative charge at the first conserved carboxyl-terminal phosphorylation position, Thr-641, in regulating the function and subcellular localization of protein kinase C betaII. Mutation of this residue to Ala results in compensating phosphorylations at adjacent sites, so that a triple Ala mutant was required to address the function of phosphate at Thr-641. Biochemical and immunolocalization analyses of phosphorylation site mutants reveal that negative charge at this position is required for the following: 1) to process catalytically competent protein kinase C; 2) to allow autophosphorylation of Ser-660; 3) for cytosolic localization of protein kinase C; and 4) to permit phorbol ester-dependent membrane translocation. Thus, phosphorylation of Thr-641 in protein kinase C betaII is essential for both the catalytic function and correct subcellular localization of protein kinase C. The conservation of this residue in every protein kinase C isozyme, as well as other members of the kinase superfamily such as protein kinase A, suggests that carboxyl-terminal phosphorylation serves as a key molecular switch for defining kinase function.

Membrane-targeting sequences on AKAP79 bind phosphatidylinositol-4, 5-bisphosphate.

Protein kinases and phosphatases are targeted through association with anchoring proteins that tether the enzymes to subcellular structures and organelles. Through in situ fluorescent techniques using a Green Fluorescent Protein tag, we have mapped membrane-targeting domains on AKAP79, a multivalent anchoring protein that binds the cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and protein phosphatase 2B, calcineurin (CaN). Three linear sequences termed region A (residues 31-52), region B (residues 76-101) and region C (residues 116-145) mediate targeting of AKAP79 in HEK-293 cells and cortical neurons. Analysis of these targeting sequences suggests that they contain putative phosphorylation sites for PKA and PKC and are rich in basic and hydrophobic amino acids similar to a class of membrane-targeting domains which bind acidic phospholipids and calmodulin. Accordingly, the AKAP79 basic regions mediate binding to membrane vesicles containing acidic phospholipids including phosphatidylinositol-4, 5-bisphosphate [PtdIns(4,5)P2] and this binding is regulated by phosphorylation and calcium-calmodulin. Finally, AKAP79 was shown to be phosphorylated in HEK-293 cells following stimulation of PKA and PKC, and activation of PKC or calmodulin was shown to release AKAP79 from membrane particulate fractions. These findings suggest that AKAP79 might function in cells not only as an anchoring protein but also as a substrate and effector for the anchored kinases and phosphatases.

Regulation of the AKAP79-protein kinase C interaction by Ca2+/Calmodulin.

The A kinase-anchoring protein AKAP79 coordinates the location of the cAMP-dependent protein kinase (protein kinase A), calcineurin, and protein kinase C (PKC) at the postsynaptic densities in neurons. Individual enzymes in the AKAP79 signaling complex are regulated by distinct second messenger signals; however, both PKC and calcineurin are inhibited when associated with the anchoring protein, suggesting that additional regulatory signals must be required to release active enzyme. This report focuses on the regulation of AKAP79-PKC interaction by calmodulin. AKAP79 binds calmodulin with high affinity (KD of 28 +/- 4 nM (n = 3)) in a Ca2+-dependent manner. Immunofluorescence staining shows that both proteins exhibit overlapping staining patterns in cultured hippocampal neurons. Calmodulin reversed the inhibition of PKCbetaII by the AKAP79(31-52) peptide and reduced inhibition by the full-length AKAP79 protein. The effect of calmodulin on inhibition of a constitutively active PKC fragment by the AKAP79(31-52) peptide was shown to be partially dependent on Ca2+. Ca2+/calmodulin reduced PKC coimmunoprecipitated with AKAP79 and resulted in a 2.6 +/- 0.5-fold (n = 6) increase in PKC activity in a preparation of postsynaptic densities. Collectively, these findings suggest that Ca2+/calmodulin competes with PKC for binding to AKAP79, releasing the inhibited kinase from its association with the anchoring protein.

More on target with protein phosphorylation: conferring specificity by location.

One important regulatory mechanism in the control of phosphorylation events is the subcellular location of phosphatases of kinases. Several serine/threonine phosphatases and kinases have now been shown to be associated with targeting subunits; these contribute to the organization and specificity of signal transduction pathways by favoring the accessibility of their enzymes to certain substrates.

Coordination of three signaling enzymes by AKAP79, a mammalian scaffold protein.

Multivalent binding proteins, such as the yeast scaffold protein Sterile-5, coordinate the location of kinases by serving as platforms for the assembly of signaling units. Similarly, in mammalian cells the cyclic adenosine 3',5'-monophosphate-dependent protein kinase (PKA) and phosphatase 2B [calcineurin (CaN)] are complexed by an A kinase anchoring protein, AKAP79. Deletion analysis and binding studies demonstrate that a third enzyme, protein kinase C (PKC), binds AKAP79 at a site distinct from those bound by PKA or CaN. The subcellular distributions of PKC and AKAP79 were similar in neurons. Thus, AKAP79 appears to function as a scaffold protein for three multifunctional enzymes.

Chicken smooth muscle myosin light chain kinase is acetylated on its NH2-terminal methionine.

The reported cDNA structure of chicken smooth muscle myosin light chain kinase (smMLCK) encodes a protein of 972 residues (Olson et al. Proc. Natl. Acad. Sci USA, 87:2284-2288, 1990). The calculated M(r) is 107,534 whereas the estimate by SDS-PAGE is approximately 130,000. Gibson and Higgins (DNA Sequence (in press)) have recently reported the possibility of errors in the cDNA sequence for non-muscle MLCK and that the NH2-terminus of both it and smMLCK may extend beyond the reported coding region. The native smMLCK is NH2-terminally blocked. A CNBr peptide derived from smMLCK contains the NH2-terminal sequence Asp-Phe-Arg-Ala corresponding to residues 2 to 4 in the smMLCK sequence indicating that Met-1 is present. Using a limited thermolysin digest we isolated an NH2-terminally blocked peptide by reversed-phase HPLC. This thermolytic peptide had a mass of approximately 797 by time of flight mass spectrometry. Amino acid analysis and Edman sequencing of a CNBr-subfragment of the thermolytic peptide indicated that it had the composition and sequence, (Met)-Asp-Phe-Arg-Ala-Asn, with a calculated mass of 753. The difference in mass corresponds to the NH2-terminal Met being blocked by acetylation. The results demonstrate that the NH2-terminal sequence of smMLCK inferred from the reported cDNA sequence is correct and that the proposed initiating Met is not removed, but modified by alpha-NH2 acetylation of the translation product.

Role of the pseudosubstrate sequence in smooth muscle myosin light chain kinase thermal stability.

Smooth muscle myosin light chain kinase (MLCK) is stable in the presence of Ca2+/calmodulin and does not undergo inactivation as reported for skeletal muscle MLCK (Kennelly, P.J., Starovasnik, M.A., Edelman, A.M., and Krebs, E.G. (1990) J. Biol. Chem. 265, 1742-1749). The 61-kDa tryptic fragment of smMLCK-(283-779) with the pseudosubstrate/calmodulin binding sequence deleted undergoes rapid inactivation (t1/2 = 5 min at 25 degrees C). Thermal inactivation renders the 61-kDa fragment more susceptible to cleavage by trypsin. The pseudosubstrate sequence, smMLCK-(787-807) prevents inactivation with high potency (half-maximal protective concentration, PC0.5 = 102 +/- 9 nM). The hexapeptide smMLCK-(797-802), Arg-Arg-Lys-Trp800-Gln-Lys, protected with a similar potency (PC0.5 = 73 +/- 14 nM). The four basic residues as well as Trp were important for maintaining protection by the hexapeptide smMLCK-(797-802). Substitution of Trp800 with Ala or Leu increased the PC0.5 to 500 nM. However, substitution of both aromatic residues Tyr794 and Trp800 in the longer pseudosubstrate peptide-(787-807) had little effect, indicating that with the longer peptide other multiple interactions were sufficient to stabilize the enzyme. The peptide substrate MLC-(11-23) A14,15 was also protective (PC0.5 = 380 nM) as was Mg(2+)-ATP, Mg(2+)-ADP, and Mg2+ plus adenosine. The results demonstrate that the sequence extending from 787-815 encoding the previously identified overlapping pseudosubstrate and calmodulin binding sequences also contains residues that are essential for maintaining thermal stability but these exhibit distinct structure/function relationships.