Functional and regulatory importance of calcium-mediated hydrophobic regions of calmodulin, protein kinase C, and other calcium-binding proteins

Slow binding kinetics of secreted protein, acidic, rich in cysteine-VEGF interaction limit VEGF activation of VEGF receptor 2 and attenuate angiogenesis

VEGF-A (VEGF) drives angiogenesis through activation of downstream effectors to promote endothelial cell proliferation and migration. Although VEGF binds both VEGF receptor 1 (R1) and receptor 2 (R2), its proangiogenic effects are attributed to R2. Secreted protein, acidic, rich in cysteine (SPARC) is a matricellular glycoprotein thought to inhibit angiogenesis by preventing VEGF from activating R1, but not R2. Because R2 rather than R1 mediates proangiogenic activities of VEGF, the role of human SPARC in angiogenesis was reevaluated. We confirm that association of SPARC with VEGF inhibits VEGF-induced HUVEC adherence, motility, and proliferation in vitro and blocks VEGF-induced blood vessel formation ex vivo. SPARC decreases VEGF-induced phosphorylation of R2 and downstream effectors ERK, Akt, and p38 MAPK as shown by Western blot and/or phosphoflow analysis. Surface plasmon resonance indicates that SPARC binds slowly to VEGF (0.865 ± 0.02 × 10(4) M(-1) s(-1)) with a Kd of 150 nM, forming a stable complex that dissociates slowly (1.26 ± 0.003 × 10(-3) s(-1)). Only domain III of SPARC binds VEGF, exhibiting a 15-fold higher affinity than full-length SPARC. These findings support a model whereby SPARC regulates angiogenesis by sequestering VEGF, thus restricting the activation of R2 and the subsequent activation of downstream targets critical for endothelial cell functions.

Drosophila UNC-13 is essential for synaptic transmission

The UNC-13 protein family has been suggested to be critical for synaptic vesicle dynamics based on its interactions with Syntaxin, Munc-18 and Doc 2alpha. We cloned the Drosophila homolog (Dunc-13) and characterized its function using a combination of electrophysiology and ultrastructural analyses. Dunc-13 contained a C1 lipid-binding motif and two C2 calcium-binding domains, and its expression was restricted to neurons. Elimination of dunc-13 expression abolished synaptic transmission, an effect comparable only to removal of the core complex proteins Syntaxin and Synaptobrevin. Transmitter release remained impaired under elevated calcium influx or application of hyperosmotic saline. Ultrastructurally, mutant terminals accumulated docked vesicles at presynaptic release sites. We conclude that Dunc-13 is essential for a stage of neurotransmission following vesicle docking and before fusion.

Phosphorylation of calmodulin by plasma-membrane-associated protein kinase(s)

Plasma-membrane-associated protein kinase(s) from normal rat liver phosphorylates exogenous bovine brain calmodulin in the absence of Ca2+ and in the presence of histone or poly(L-lysine). Maximum levels of calmodulin phosphorylation are obtained at a poly(L-lysine)/calmodulin molar ratio of 0.4. Phosphoamino acid analysis revealed that calmodulin is phosphorylated on serine, threonine and tyrosine residues. Endogenous plasma-membrane-associated calmodulin was also phosphorylated by plasma-membrane-associated protein kinase(s) in the absence of added cationic protein or polypeptide. The identity of endogenous phosphocalmodulin was confirmed by immunoprecipitation with a specific anti-calmodulin monoclonal antibody. Ehrlich ascites tumor cell plasma membranes do not contain endogenous calmodulin. However, membrane-associated protein kinase(s) from these tumor cells phosphorylates bovine brain calmodulin in the presence of poly(L-lysine). These data demonstrate that phosphocalmodulin is present in liver plasma membranes and suggest that this post-translational modification could have a physiological role in this location.

Calcium-supported calpain degradation rates for cardiac myofibrils in diabetes. Sulfhydryl and hydrophobic interactions

OBJECTIVE

The purpose was to investigate the calcium required for calpain-mediated degradation of selected cardiac myofibril proteins modified by diabetes, sulfhydryl (SH) and hydrophobic reagents.

METHODS

After 20 weeks of streptozotocin-induced (55 mg.kg-1) diabetes, calcium sensitive calpain (1.5 U.ml-1) degradation rates of purified cardiac myofibrillar proteins (1 mg.ml-1) were measured, in vitro, and compared to degradation rates for N-ethylmaleimide (NEM) and 2-p-toluidinylnapthalene-6-sulfonate (TNS) treated samples.

RESULTS

Diabetes (blood glucose of 550 +/- 32 mg.dl-1) reduced the yield of purified myofibrillar protein with minimal change in fibril protein composition. Total SH group reactivities (nmol.mg-1.30min) were 220 +/- 21, 163 +/- 17 and 156 +/- 24 for control, diabetic and NEM-treated (0.5 mM) myofibrils (p < or = 0.05). Calpain degradation rates were faster for all diabetic and SH modified myofibrillar proteins (p < or = 0.05), with a 45 and 35% reduction in the pCa50 for a 37 kDa protein of diabetic and NEM-treated fibril complexes. For control myofibrils, both 100 and 200 uM TNS, reduced calpain degradation rates to a similar extent for all substrate proteins. In contrast, diabetic and NEM-treated samples showed a further reduction in calpain degradation rates with increasing TNS from 100 to 200 uM.

CONCLUSION

Our results support the hypothesis that in diabetes the calcium requirements for calpain degradation rates are reduced and dependent upon sulfhydryl group status and Ca(2+)-induced hydrophobic interactions, implicating a 37 kDa myofbillar-complexed protein.

Irreversible oxidative inactivation of protein kinase C by photosensitive inhibitor calphostin C

Isolated protein kinase C (PKC) was irreversibly inactivated by a brief (min) incubation with calphostin C in the presence of light. This inactivation required Ca2+ either in a millimolar range in the absence of lipid activators or in a submicromolar range in the presence of lipid activators. In addition, an oxygen atmosphere was required suggesting the involvement of oxidation(s) in this inactivation process. Furthermore, PKC inactivation might involve a site-specific oxidative modification of the enzyme at the Ca(2+)-induced hydrophobic region. Physical quenchers of singlet oxygen such as lycopene, beta-carotene, and alpha-tocopherol all reduced the calphostin C-induced inactivation of PKC. In intact cells treated with calphostin C, the inactivation of PKC was rapid in the membrane fraction compared to cytosol. This intracellular PKC inactivation was also found to be irreversible. Therefore, calphostin C can bring prolonged effects for several hours in cells treated for a short time. Taken together these results suggest that the calphostin C-mediated inactivation of PKC involves a site-specific and a 'cage' type oxidative modification of PKC.

Novel Ca2+-activated neutral protease from an aquatic fungus, Allomyces arbuscula

A Ca2+-activated neutral protease was purified to homogeneity from an aquatic Phycomycete fungus, Allomyces arbuscula. It requires millimolar concentrations of Ca2+ for activation (1.8 to 2 mM for 50% activation). Sr2+ can replace Ca2+ but at higher concentrations (4 mM for 50% activation). The enzyme is a dimer of 40-kilodalton subunits and contains six cysteine residues, three of which are revealed only after the addition of micromolar concentrations of Ca2+; the other three are free. Enzyme activity is strongly inhibited by SH-group inhibitors and some trypsin inhibitors (leupeptin and alpha-N-tosyl-L-lysine chloromethyl ketone). The enzyme lacks general trypsinlike specificity, since substrates containing tryptic cleavage sites are not cleaved nor is enzyme activity inhibited by other trypsin inhibitors. The enzyme has many functional similarities to the extensively characterized mammalian and avian Ca2+-activated neutral proteases but differs in its substrate specificity, inhibition by alpha-N-tosyl-L-phenylalanine chloromethyl ketone, and subunit structure. It is, nevertheless, presumed that this enzyme has a similar high order of specificity and is involved in the regulation of a specific growth function.

Susceptibility of protein kinase C to oxidative inactivation: loss of both phosphotransferase activity and phorbol diester binding

Exposure of protein kinase C to low concentrations of either N-chlorosuccinimide or H2O2 resulted in rapid and parallel loss of phosphotransferase activity and phorbol ester binding. This oxidative inactivation of protein kinase C also occurred in intact cells exposed to a low concentration of H2O2. With H2O2 treatment the rate of inactivation of protein kinase C in the cytosol of MCF-7 cells was rather slower than that which occurred in the cytosol of PYS cells. However, in both cell types, the oxidative inactivation of membrane-associated protein kinase C occurred rapidly in comparison to the enzyme in the cytosol. Prior treatment of cells with phorbol ester to induce membrane association (stabilization) of protein kinase C, followed by exposure to H2O2, resulted in increased inactivation of protein kinase C, suggesting that membrane association of protein kinase C increases its susceptibility to oxidative inactivation.