Ykt6p, a Prenylated SNARE Essential for Endoplasmic Reticulum-Golgi Transport*

Vesicular transport between secretory compartments requires specific recognition molecules called SNAREs. Here we report the identification of three putative SNAREs, p14 (Sft1p), p28 (Gos1p), and a detailed characterization of p26 (Ykt6p). All three were originally isolated as interacting partners of the cis Golgi target membrane-associated SNARE Sed5p, when Sec18p (yeast NSF) was inactivated. YKT6 is an essential gene that codes for a novel vesicle-associated SNARE functioning at the endoplasmic reticulum-Golgi transport step in the yeast secretory pathway. Depletion of Ykt6p results in the accumulation of the p1 precursor (endoplasmic reticulum form) of the vacuolar enzyme carboxypeptidase Y and morphological abnormalities consistent with a defect in secretion. Membrane localization of Ykt6p is essential for protein function and is normally mediated by isoprenylation. However, replacement of the isoprenylation motif with a bona fide transmembrane anchor results in a functional protein confirming that membrane localization, but not isoprenylation per se, is required for function. Ykt6p and its homologues are highly conserved from yeast to human as demonstrated by the functional complementation of the loss of Ykt6p by its human counterpart. This is the first example of a human SNARE protein functionally replacing a yeast SNARE. This observation implies that the specific details of the vesicle targeting code, like the genetic code, are conserved in evolution.

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