The vacuolar
protein sorting (VPS) pathway is required for transport of
proteins from the Golgi to the vacuole. Mutations in class E
VPS genes disrupt this process and result in accumulation of
vacuolar proteins in the prevacuolar compartment (PVC). Although class
E Vps proteins are required for protein transport out of the PVC, the
mechanism by which they do so is not understood. We show here that
inactivation of a new member of the class E VPS genes,
COS1, results in a prototypic class E phenotype. We have
localized Cos1p to the PVC by indirect immunofluorescence, and are
currently determining the role of Cos1p myristoylation in
localization. Several other class E Vps proteins have been localized
to the PVC, where Emr and colleagues proposed that they are present
and function as a large protein complex. Consistent with this
hypothesis, 2-hybrid results suggest that Cos1p interacts with several
other class E Vps proteins including Vps36p, Vps28p, and
Stp22p. Interestingly, protein interaction is necessary, but not
sufficient, for function because carboxyl terminal deletions of
COS1 interact normally with Vps36p and Vps28p but confer a
dominant-negative phenotype. Finally, Cos1p is efficiently localized
in strains lacking any one of several class E vps genes (eg.
VPS36, VPS27, VPS24) genes, suggesting protein
interaction does not play a critical role in localization. Further
characterization of Cos1p may help elucidate the mechanism of
trafficking through the PVC.
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