CALCIUM MOBILIZATION IS REQUIRED FOR NUCLEAR VESICLE FUSION IN-VITRO - IMPLICATIONS FOR MEMBRANE TRAFFIC AND IP(3) RECEPTOR FUNCTION

We studied the fusion of nuclear vesicles bound to chromatin in Xenopu s egg extracts. Fusion was inhibited by 5 mM BAPTA, a Ca2+ buffer that suppresses cytosolic [Ca2+] gradients. The BAPTA-inhibited step in fu sion was biochemically distinct from, and occurred later than, the GTP gammaS-sensitive step mediated by the monomeric GTPase, ADP-ribosylati on factor. Exogenous inositol 1,4,5-trisphosphate (IP3), which trigger s Ca2+ release from lumenal stores via IP3 receptors, stimulated fusio n in the presence of BAPTA. This rescue was specific, because inositol 1,3,4-trisphosphate had no effect. Heparin, a potent antagonist of IP 3 receptors, independently blocked fusion in an IP3-reversible manner. We suggest that phosphoinositide signaling may regulate nuclear vesic le fusion.