Summary

In Saccharomyces cerevisiae cells lacking SHR3, amino acid permeases do not enter into COPII transport vesicles and specifically accumulate in the membrane of the endoplasmic reticulum. Shr3p functions as a packaging chaperone to prime transport vesicle formation in the proximity of amino acid permeases. A genetic screen was developed that enabled the Schizosaccharomyces pombe SHR3 functional homologue, designated psh3(+) (pombe SHR3), to be cloned. The psh3(+) gene encodes a protein of 215 amino acids, which shares a high degree of structural and functional similarity with Shr3p. The heterologous expression of psh3(+) complements many, but not all, shr3 null mutant phenotypes in S. cerevisiae in a temperature-dependent manner. Psh3p is localised to the endoplasmic reticulum of S. pombe cells, and strains lacking the psh3(+)gene exhibit decreased rates of amino acid uptake due to reduced levels of functional permeases in the plasma membrane. No packaging chaperones, or proteins exhibiting homology with packaging chaperones, have so far been identified in other eukayotic organisms. The findings reported here are the first to establish that specific packaging chaperones exist in divergent organisms, and demonstrate a conserved function of packaging chaperones in facilitating the export of large polytopic membrane proteins from the endoplasmic reticulum.