EFFECTS OF SYNAPTIC VESICLES ON ACTIN POLYMERIZATION

We have analyzed the effects of synaptic vesicles on actin polymerizat ion by using a time-resolved spectrofluorometric assay, We have found that synaptic vesicles have complex effects on the kinetics of actin p olymerization, which vary depending on whether the synaptic vesicle-sp ecific phosphoprotein synapsin I is absent or present on their membran e, Synapsin I bound either to synaptic vesicles or to pure phospholipi d vesicles exhibits phosphorylation-dependent actin-nucleating activit y, Synaptic vesicles depleted of endogenous synapsin I decrease the ra te and the final extent of actin polymerization, an effect ,which is n ot observed with pure phospholipid vesicles. Thus, the state of associ ation of synapsin I with synaptic vesicles, which is modulated by its state of phosphorylation, may affect actin assembly and the physico-ch emical characteristics of the synaptic vesicle microenvironment.