For example, it has been argued that syt III is a major Ca2+ sensor in PC12 cells because the C2A domain derived from this isoform is potent inhibitor of release (Sugita et al., 2002). activity. Moreover, we measured the expression levels of endogenous syts in PC12 cells; the major isoforms are I and IX, with trace levels of VII. As expected, if syts I and IX function as Ca2+ sensors, fragments from these isoforms blocked secretion. These data suggest that syts trigger fusion via their Ca2+-regulated interactions with t-SNAREs and PIP2, target molecules known to play critical roles in exocytosis. syt I are associated with a loss of SDS-resistant SNARE complexes (Littleton et al., 2001), further supporting the idea that syt I is required for the assembly of stable SNARE complexes. A recent study using mutant versions of C2A-I and C2A-VII indicated that inhibition of catecholamine release from cracked PC12 cells was correlated with the Ca2+ requirements for binding to PS (Shin et al., 2002); C2A domains with high apparent Ca2+ affinities inhibited SB-742457 release, whereas those with lower affinities did not. This led the authors to suggest that syt functions during exocytosis by interacting with PS rather than with SNARE proteins. However, in these experiments C2 domains with relatively low apparent affinities for Ca2+ did not become inhibitory at Ca2+ concentrations that were sufficient to drive efficient binding to PS SB-742457 (Shin et al., 2002), arguing that C2ACPS relationships cannot mediate inhibition. Also, in our experiments the Ca2+-induced PS-binding activity of all of the C2 domains was saturated with Ca2+, yet some PS-binding C2 domains failed to inhibit launch (Fig. 1 B). Moreover, we found that C2A-I, a noninhibitory website, and C2A-III and -VII, which are both potent inhibitors of launch, bind PS-containing liposomes to the same degree, and with the same dissociation constants, at Ca2+ concentrations identical to those used in our secretion assays (Fig. 2, E and F). These data clearly demonstrate that PS-binding activity is not Rabbit polyclonal to PCDHB10 sufficient to block launch under the conditions of our assay system. Finally, isolated C2B domains from syts I and IX lack strong PS-binding activity (Fig. 6 A) yet inhibit catecholamine launch from Personal computer12 cells (Fig. 5). In summary, PS-binding activity is definitely neither necessary nor adequate to block launch. It is possible that higher concentrations of PS-binding C2 domains could block exocytosis in our assay system. However, we note that a 100-nm liposome that contains 25% PS can bind 225 syt molecules (Bai and Chapman, 2003). PS is definitely a ubiquitous and abundant constituent of synaptosomal membranes (7C18 mol%; Breckenridge et al., 1972; Westhead, 1987). Consequently, Personal computer12 cells (10 m in diameter) harbor a large number of C2 domainCbinding sites, making them hard to saturate without having nonspecific effects on other relationships within cracked cells. In contrast, PIP2 is present at 10-fold lower concentrations in cellular membranes (Tran et al., 1993), and we estimate that in our launch assays t-SNAREs are present at sub M concentrations (0.4 M SNAP-25 and 0.06 M syntaxin; data unpublished). Therefore, the levels of free PIP2 and t-SNAREs are likely to be strongly affected by addition of M concentrations of recombinant C2 domains. What then is the in vivo function of the noninhibitory PS-binding C2A domains? Our quantitative immunoblots demonstrate that syts I and IX are the major Ca2+ detectors in Personal computer12 cells (Fig. 4), yet their isolated C2A domains fail to inhibit launch. The first idea as to the function of C2A-I was provided by genetic studies in flies is definitely unique from wild-type or syt-null flies (DiAntonio and Schwarz, 1994; Littleton et al., 1994). exhibits some SB-742457 degree of synchronous launch that is absent from null mutants (Yoshihara and Littleton, 2002). SB-742457 These data suggest that the part of C2A is definitely to accelerate fusion. Other organizations possess mutated the C2A website, but a consensus as to the effects of these mutations has not been reached (Fernndez-Chcon et al., 2001, 2002; Robinson et al., 2002). In our acute disruption experiments, inhibition of secretion could involve blockade of additional sytCeffector interactions. For example, the ability of C2B-I to inhibit secretion from cracked cells is likely to also involve perturbation of native syt oligomerization (Desai et al., 2000; Littleton et al.,.