EFFECTS OF COA AND ACYL-COAS ON GTP-DEPENDENT CA2+ RELEASE AND VESICLE FUSION IN RAT-LIVER MICROSOMAL VESICLES

(1) CoA (IC50 23 muM) and acyl-CoAs (IC50 values 15-18 muM) inhibit GTP-dependent vesicle fusion in rat liver microsomal vesicles. Acyl-CoAs of carbon chain length C8 and C20 are much less effective than acyl-CoAs of carbon chain length C-14-C18. The effect of CoA is mimicked by dephospho-CoA, but not by desulpho-CoA. High acyl-CoA concentrations (50 muM) appear to favour formation of small vesicles (budding), while 50 muM CoA does not. (2) Low concentrations of CoA (EC50 2 muM) and palmitoyl-CoA (10 muM) cause re-accumulation of Ca2+ released in response to GTP. This re-accumulation is into an Ins(1,4,5)P3-sensitive sensitive compartment. By investigation of the effects of CoA and palmitoyl-CoA on the thapsigargin-induced passive leak rate of Ca2+, and on the latency of the mannose-6-phosphatase of the vesicles, we conclude that CoA and palmitoyl-CoA cause decreased vesicle permeability rather than stimulation of Ca2+ pumping activity. (3) It is suggested that GTP-induced membrane fusion in rat liver microsomes involves an as yet uncharacterized acylation-deacylation reaction which is required to produce complete vesicle sealing.