Calcium mobilization is required for nuclear vesicle fusion in vitro: implications for membrane traffic and IP3 receptor function.

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