Golgi-resident glycosyltransferases are a family of enzymes that sequentially modify glycoproteins in a subcompartment-specific manner. These type II integral membrane proteins are characterized by a short cytoplasmically exposed amino-terminal tail and a luminal enzymatic domain. The cytoplasmic tails play a role in the localization of glycosyltransferases, and coat protein complex I (COPI) vesicle-mediated retrograde transport is also involved in their Golgi localization. However, the tails of these enzymes lack known COPI-binding motifs. Here, we found that Vps74p bound to a pentameric motif present in the cytoplasmic tails of the majority of yeast Golgi-localized glycosyltransferases, as well as to COPI. We propose that Vps74p maintains the steady-state localization of Golgi glycosyltransferases dynamically, by promoting their incorporation into COPI-coated vesicles.
Ykt6p is a nonsyntaxin SNARE implicated in multiple intracellular membrane trafficking steps. Here we present the structure of the NH2-terminal domain of Ykt6p (Ykt6pN, residues 1 to 140). The structure of Ykt6pN differed entirely from that of syntaxin and resembled the overall fold of the actin regulatory protein, profilin. Like some syntaxins, Ykt6p adopted a folded back conformation in which Ykt6pN bound to its COOH-terminal core domain. The NH2-terminal domain plays an important biological role in the function of Ykt6p, which in vitro studies revealed to include influencing the kinetics and proper assembly of SNARE complexes.
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