, 2002), and subsequently identified as an active zone protein and renamed CAST (Ohtsuka et al., 2002) or ERC (Wang et al., 2002). The field agreed on the original ELKS name for the protein, although the CAST and ERC names are still used occasionally. ELKS consist largely of predicted coiled-coil sequences with no apparent domain structure. The mammalian genome contains two ELKS genes encoding structurally similar proteins, whereas C. elegans expresses a single ELKS gene highly homologous to mammalian ELKS. Mammalian ELKS genes contain alternative N-terminal promoters and alternatively spliced C-terminal sequences,

with a shorter C-terminal sequence that is primarily expressed in brain and a longer C-terminal sequence that is primarily expressed in peripheral tissues ( Wang et al., 2002 and Kaeser et al., 2009). In contrast to other organisms, Drosophila expresses an ELKS fusion protein called “bruchpilot” (German for buy GDC-0941 “crash pilot”) that consists of an N-terminal ELKS-related domain and a C-terminal plectin-related domain ( Wagh et al., 2006). As documented by its repeated rediscovery, ELKS likely functions in several cellular processes, Docetaxel and engages in multiple protein-protein interactions (Figure 2). It binds to Rab6 in a GTP-dependent

manner, implicating it in membrane traffic involving the trans-Golgi complex (Monier et al., 2002). Its active zone localization was discovered by virtue of its binding to the RIM PDZ domains (Wang et al., 2002). The C terminus of ELKS probably also binds to other PDZ domain proteins, as described

for syntenin-1 (Ko et al., 2006), and ELKS furthermore directly binds to α-liprins (Ko et al., 2003a; see discussion above). Although initial overexpression and peptide injection experiments suggested a major function for ELKS2 in neurotransmitter release (Takao-Rikitsu et al., 2004), deletion of ELKS in C. elegans and of ELKS2 in mice did not impair neurotransmitter release ( Deken et al., 2005 and Kaeser et al., Cell press 2009). Interestingly, however, ELKS was required in C. elegans for the ability of the α-liprin gain-of-function mutation to suppress the syd-1 mutation ( Dai et al., 2006; see discussion above). This result shows that at least for synapse formation and function under basal conditions, the synaptic function of ELKS is dispensible. Moreover, although acute or constitutive deletion of ELKS2 in mice did not produce a decrease in neurotransmitter release, they caused an increase in the readily releasable pool of synaptic vesicles ( Kaeser et al., 2009). In contrast, constitutive deletion of ELKS1 caused embryonic lethality in mice, suggesting that the protein is essential for survival in a nonneuronal function (P.S. Kaeser and T.C.S., unpublished data). At first glance, ELKS appears to have a more important function in Drosophila where deletion of bruchpilot produces a loss of the t bars characteristic of Drosophila synapses ( Wagh et al., 2006).