An enzyme that can be included into the so-called conventional PKCs has been purified to homogeneity from the mantle tissue of the sea mussel Mytilus galloprovincialis. This enzyme has a molecular weight of 60 kDa, which is DAG-dependent, PS-activated, and Ca2+-dependent. It was separated from a Ca2+-independent PKC (p105) (Mercado et al., Mol Cell Biochem 233:99-105, 2002) by means of an ionic exchange chromatography on DE-52 cellulose. The molecular weights and kinetic properties of both the enzymes are different. The protein p60 is broadly distributed among the tissues, which suggests that it may carry out specific functions, different from those performed by p105.

No abstract

The innate immune system of marine mussels (Mytilus galloprovincialis) is operated by phagocytic cells termed hemocytes. Lipopolysaccharide (LPS), interleukin-2 (IL-2), or platelet-derived growth factor (PDGF) increase biogenic amine synthesis in these cells, and the enzymes Ca(2+)-independent protein kinase C (PKC) (p105/108) and Ca(2+)-dependent PKC (p60) are involved in these processes. Stimulation by PDGF induces a down-regulation process affecting the form p108 of the Ca(2+)-independent PKC. In addition, PDGF produces the increase of expression of p60 in the membrane fraction. IL-2 induces the disappearance of p108 from the membrane but does not affect the presence of p60 in cytosol or membrane. For its part, LPS activates exclusively p60 by a down-regulation mechanism. The ensemble of results suggests that each agonist starts a pathway that implicates the PKC isoenzymes that mediate the regulation of the activities dopa decarboxylase, dopamine beta-hydroxilase, and phenyletanolamine N-methyltranferase, which lead to different actions related to biogenic amine synthesis.

Previous works revealed the presence of a Ca(2+)-dependent protein kinase (p60) and a Ca(2+)-independent protein kinase (p105) in the mantle tissue from the sea mussel Mytilus galloprovincialis Lmk. The expression of both isoforms shows a balance between cytosolic and membrane fractions in mantle, gills, and hepatopancreas, whereas, in hemocytes, their expression is mainly cytosolic, as happens in muscle tissues with p60 alone. Both enzymatic forms contain phosphorylated serines, and no phosphorylation was detected in tyrosines. Only the form p105 mediates the PMA-induced activation of the hemocytes of M. galloprovincialis, and it does so by a process of down-regulation. The form p60 does not respond to the presence of the phorbol ester, suggesting structural differences related to the binding sites of the diacylglycerol.

Experimental evidence shows that the hemocytes of Mytilus galloprovincialis Lamark have a multivalent receptor that responds to diverse agonists provoking the activation of the immune response. The same such agonists have the property of modulating the operability of the endocrine system. The relationship between the neuroendocrine and immune systems has led to the consideration of hemocytes as a mobile immune-brain. In Mytilus galloprovincialis, two protein kinases C (PKC) mediate these processes. According to their kinetic and regulatory properties, they were classified into the families of the Ca 2+ -independent PKCs (nPKC) and Ca 2+ -dependent PKCs (cPKC), and named after their molecular masses, p105 and p60, respectively. Lipopolysaccharide (LPS), interleukin-2 (IL-2) and platelet-derived growth factor (PDGF) induce catecholamine synthesis but display an agonist-specific picture. The particular responses of the diverse PKC isozymes may cause the differences. These results would agree with p60 acting only on dopamine synthesizing enzyme, whereas p105 would affect norepinephrine and epinephrine production. The action of IL-2 or PDGF is related to the regulation of mussel PKC activities through the induction of mechanisms of down-regulation and balancing of p105 phosphorylation levels. As proof of the stress response, the agonists assayed induce catecholamine synthesis faster than the immune response they provoke. Also, the seasonal behavior of both processes differs in time, which proves their different timings.