INHIBITION OF NUCLEAR VESICLE FUSION BY ANTIBODIES THAT BLOCK ACTIVATION OF INOSITOL 1,4,5-TRISPHOSPHATE RECEPTORS

Inositol 1,4,5-trisphosphate (IP3) receptors are ligand-gated channels that release intracellular Ca2+ stores in response to the second mess enger, IP3. We investigated the potential role of IP3 receptors during nuclear envelope assembly in vitro, using Xenopus egg extracts. Previ ous work suggested that Ca2+ mobilization is required for nuclear vesi cle fusion and implicated IP3 receptor activity. To test the involveme nt of IP3 receptors using selective reagents, we obtained three distin ct polyclonal antibodies to the type 1 IP3 receptor. Pretreatment of m embranes with two of the antibodies inhibited IP3-stimulated Ca2+ rele ase in vitro and also inhibited nuclear vesicle fusion. One inhibitory serum was directed against 420 residues within the ''coupling'' domai n, which includes several potential regulatory sites. The other inhibi tory serum was directed against 95 residues near the C terminus and id entifies an inhibitory epitope(s) in this region. The antibodies had n o effect on receptor affinity for IP3. Because nuclear vesicle fusion was inhibited by antibodies that block Ca2+ flux, but not by control a nd preimmune antibodies, we concluded that the activation of IP3 recep tors is required for fusion. The signal that activates the channel dur ing fusion is unknown.