Sandwich-type binding motif

Our group also identified a second action of syntaxin 1A (but not IB) on N-type calcium channels. Co-expression of this protein with N-type calcium channels results in a tonic G protein-mediated inhibition of channel activity that does not involve receptor activation (Jarvis et al. 2000). This appears to be due to a syntaxin-mediated co-localization of the channel and Gpy that ultimately culminates in tonic channel inhibition (Figure 3ii). Indeed, syntaxin 1A binds to Gpy in vitro on a region that is distinct from the Gpy site involved in interactions with the synprint motif. These data are consistent with the idea of a sandwich formed by the channel, syntaxin and Gpy (Jarvis et al. 2002). Intriguingly, although syntaxin IB can also bind to Gpy in vitro, no tonic G protein inhibition of channel activity ensues (Lu et al. 2001). This suggests that, compared with syntaxin 1A, syntaxin IB adopts a slightly different channel-bound conformation. The syntaxin-mediated effect on G protein inhibition appears to be much more robust than the effect on channel... 

The first member of the GT-A superfamily fold was identified in 1999 when the three-dimensional stmcture of SpsA was reported (19). This enzyme from Bacillus subtilis is involved in the formation of the spore coat and is a member of the Leloir type of GTs given its apparent use of nucleotidyl-diphospho-donor sugars as a substrate. Enzymes with the GT-A fold have an N-terminal a/p/a sandwich motif that resembles a Rossmann motif and is involved in nucleotide donor binding. Most GT-A fold enzymes also have a characteristic Asp-Xxx-Asp (DXD) or equivalent motif (EXD or TDD) near the center of the protein that coordinates to the phosphate in nucleotide donors via a divalent metal cation (Mn + or Mg +). These motifs are present in the structure of the bovine pi,4-galactosyltransferase I (5) (Fig. 4a). 

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