INOSITOL-1,3,4,5-TETRAKISPHOSPHATE BINDING TO C2B DOMAIN OF IP4BP SYNAPTOTAGMIN-II

IP4BP/Synaptotagmin II is an inositol-1,3,4,5-tetrakisphosphate (IP4) or inositol polyphosphate-binding protein, which is accumulated at ner ve terminals. Here we report a novel function of the C2B domain, which was originally thought to be responsible for Ca2+-dependent binding t o phospholipid membranes. A study of deletion mutants showed that abou t 30 amino acids of the central region of the C2B domain of mouse IP4B P/synaptotagmin II (315 IHLMQNGKRLKKKKTTVKKK-TLNPYFNESFSF 346) are ess ential for inositol polyphosphate binding. This binding domain include s a sequence corresponding to the squid Pep20 peptide, which is also k nown to be essential for neurotransmitter release (Bommert, K., Charlt on, M. P., DeBello, W. M., Chin, G. J. Betz, H., and Augustine, G. J. (1993) Nature 363, 165-165), suggesting that inositol polyphosphate ha s some effect on neurotransmitter release. Rabphilin 3A, another neuro nal protein containing C2 domains, cannot bind IP4, indicating that th e IP4 binding property is specific to the C2B domain of synaptotagimin . Phospholipid and IP4 binding experiments clearly indicated that the C2A and C2B domains have different functions. The C2A domain binds pho spholipid in a Ca2+-dependent manner, but the C2B domain binds inosito l polyphosphate and phospholipid irrespective of the presence of Ca2+. Our data suggest that the C2B domain of synaptotagimin is the inosito l polyphosphate sensor at the synaptic vesicle and may be involved in synaptic function.