||Deletion of the VPS74 (V̲acuolar P̲rotein S̲orting 74) gene in yeast Sacchromyces cerevisiae results in a modest defect in carboxypeptidase Y (CPY) sorting. I found that null mutants of VPS74 were sensitive to Calcofluor white, indicating that the cell walls of these cells were defective. Consistent with this, processing of the GPI-anchored protein Gas1p, which is required for proper cell wall assembly and morphogenesis, was altered in vps74Δ cells. I have established that the defects of cell wall integrity and Gas1p processing observed in vps74Δ cells are attributable to the mislocalization and rapid degradation of a subset of Golgi mannosyltransferases, and in particular, Kre2p. Using a yeast two hybrid assay, I have demonstrated that Vps74p interacts with the cytoplasmic tail of Kre2p and that the FLS amino acid sequence in Kre2p is required to mediate this binding. Mammalian homologues of Vps74p, GMx33α/GPP34 and GMx33β/GPP34R, can complement the Gas1p processing and Calcofluor white sensitivity defects of vps74Δ cells to varying degrees indicating that some of Vps74p’s activities are evolutionarily conserved. My findings suggest that Vps74p functions as a protein sorting receptor mediating the retrieval and hence steady-state distribution of a subset of Golgi mannosyltransferases. To further investigate the function of VPS74, I performed a synthetic lethal screen. From this screen I identified three mutants that showed synthetic lethal interactions with cells lacking the VPS74 gene. The mutations responsible for the synthetic lethal phenotypes all mapped to a single locus - YIL039W (GPS1, G̲as1 P̲rotein S̲orting 1) an uncharacterized, non-essential yeast gene. Null, or temperature-sensitive mutations in GPS1, cause a transport delay of the GPI-anchored protein Gas1p from the endoplasmic reticulum, which suggests a role for Gps1p in ER-to-Golgi traffic. Gps1p contains a presumptive calcineurin-like phosphoesterase domain in its C-terminus and I attempted to identify candidate substrates for Gps1p by looking for dosage suppressors of the temperature-sensitive growth defect of vps74Δgps1ts cells. One such suppressor, termed DCR2 (d̲osage c̲ell cycle r̲egulator 2) also contains a calcineurin-like phosphoesterase domain, which suggests that Gps1p and Dcr2p belong to the same calcineurin-like phosphoesterase protein family. Consistent with this, cells that lack both DCR2 and GPS1 are inviable. I have established that Gps1p and Dcr2p are integral membrane proteins that localize to the ER / Golgi, and that the presumptive catalytic domains of these proteins are located in the lumen of these organelles. Characterization of the gps1-1 temperature-sensitive allele in both vps74Δ and dcr2Δ genetic backgrounds revealed a defect in cell polarity; cells show abnormal actin cytoskeleton organization, depolarized chitin deposition and fail to respond to alpha factor. In addition, I have determined that vps74Δgps1ts cells show a temperature-dependent remedial osmo-resistance phenotype under conditions that induce high osmotic stress.