Figure 9. Interaction of the N-terminus of Sso2 with Sec3 in comparison with that in other syntaxin/SM protein complexes.

(A–C) Crystal structures of the complexes of Sso2/Sec3 (PDB code: 7Q83), syntaxin-1/Munc18 (PDB code: 3C98), and Tlg2/Vps45 (PDB code: 2XHE), respectively. All N-terminal extensions of these t-SANREs are indicated by arrows. (D) Superposition of the three complex structures shown in A, B, and C. (E) Superposition of the Rho1/Sec3 complex (PDB code: 3A58) with the Sec3/Sso2 complex (PDB code: 7Q83) on top of their overlapped Sec3 components.

Figure 9—figure supplement 1. A hypothetical working model for how Sso2 recruits Sec3 in vesicle docking.

The NPY motifs at the N-terminus of Sso2 first reach out to dock into the complementary pocket on the Sec3 PH domain, which then pulls back and allows Sec3 to bind to the middle of the helical bundle of Sso2 (helices Ha, Hb and Hc are shown as semitransparent white cylinders, and H3 as an orange cylinder). Binding of Sec3 allosterically destabilizes the linker connecting Hc and H3 of Sso2 and primes downstream events in SNARE complex assembly.