Characteristics of protein kinase C-independent exocytosis in human platelets

Effects of protein kinase inhibitors on thein vitrogrowth ofBabesia bovis

Staurosporine, Ro-31-7549, and KN-93, which are inhibitors of serine/threonine protein kinase, protein kinase C, and calcium-modulin kinase, respectively, were tested for their effects on thein vitrogrowth ofBabesia bovis. Staurosporine was the most effective inhibitor, completely clearing the parasitaemia as early as the first day of exposure at a concentration of 100 μM. Moreover, staurosporine caused a significant increase in the percentage of extracellular merozoites, most likely due to the inhibition of erythrocyte invasion by the parasite. Although 5 mMRo-31-7549 and KN-93 had a suppressive action, this was not enough to destroy the parasite. Interestingly, concentrations of 0·5 to 5 mMKN-93 influenced the parasitic development within the infected erythrocytes. The present study suggests thatB. bovisrequires, to a certain extent, the phosphorylations mediated by parasite- or host erythrocyte-protein kinases, in particular, for the processes of successful invasion of erythrocytes and intraerythrocytic development.

Myristoylated alanine-rich C kinase substrate phosphorylation is involved in thrombin-induced serotonin release from platelets

Stimulation of platelets by thrombin induces protein kinase C (PKC) activation, phosphorylation of pleckstrin, aggregation and serotonin release. Here, we demonstrate that, in human platelets, thrombin stimulation also induced phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) and serotonin release in intact and digitonin-permeabilized platelets. MARCKS is known to bind actin and cross-link actin filaments, and this is inhibited by PKC-evoked MARCKS phosphorylation. MARCKS phosphorylation and serotonin release in response to increasing concentrations of thrombin have a similar EC50 and time course and, in permeabilized platelets, peptide MPSD, with an amino acid sequence corresponding to the phosphorylation site domain of MARCKS, blocked both responses. However, pleckstrin and myosin light chain phosphorylations were not modified. Ala-MPSD, in which the four serine residues of MPSD were substituted by alanines was ineffective. The results suggest a role for MARCKS in platelet secretion. The fact that pleckstrin phosphorylation has a different time course and was not modified in the presence of MPSD when MARCKS phosphorylation and serotonin release were inhibited would suggest either that pleckstrin phosphorylation is unrelated to secretion or that it might only be involved upstream in the events leading to secretion.