We have investigated the mechanism of Ca2+ entry into fura-2-loaded human platelets by preventing the prenylation of proteins such as small GTP-binding proteins. The farnesylcysteine analogues farnesylthioacetic acid (FTA) and N-acetyl-S-geranylgeranyl-L-cysteine (AGGC), which are inhibitors of the methylation of prenylated and geranylgeranylated proteins respectively, significantly decreased thrombin-evoked increases in intracellular free Ca2+ concentration ([Ca2+]i) in the presence, but not in the absence, of external Ca2+, suggesting a relatively selective inhibition of Ca2+ entry over internal release. Both these compounds and N-acetyl-S-farnesyl-L-cysteine, which had similar effects to those of FTA, also decreased Ca2+ entry evoked by the depletion of intracellular Ca2+ stores with thapsigargin. The inactive control N-acetyl-S-geranyl-L-cysteine was without effect. Patulin, an inhibitor of prenylation that is inert with respect to methyltransferases, also decreased store-regulated Ca2+ entry. Cytochalasin D, an inhibitor of actin polymerization, significantly decreased store-regulated Ca2+ entry in a time-dependent manner. Both cytochalasin D and the farnesylcysteine analogues FTA and AGGC inhibited actin polymerization; however, when evoking the same extent of decrease in actin filament formation, FTA and AGGC showed greater inhibitory effects on Ca2+ entry, indicating a cytoskeleton-independent component in the regulation of Ca2+ entry by small GTP-binding-protein. These findings suggest that prenylated proteins such as small GTP-binding proteins are involved in store-regulated Ca2+ entry through actin cytoskeleton-dependent and cytoskeleton-independent mechanisms in human platelets.
Identification of the ral and rac1 Gene Products, Low Molecular Mass GTP-binding Proteins from Human Platelets
