SCD5, a suppressor of clathrin deficiency, encodes a novel protein with a late secretory function in yeast

Clathrin and its associated proteins constitute a major class of coat proteins involved in vesicle budding during membrane transport. An interesting characteristic of the yeast clathrin heavy chain gene (CHC1) is that in some strains a CHC1 deletion is lethal, while in others it is not. Recently, our laboratory developed a screen that identified five multicopy suppressors that can rescue lethal strains of clathrin heavy chain-deficient yeast (Chc scd1-i) to viability. One of these suppressors, SCD5, encodes a novel protein of 872 amino acids containing two regions of repeated motifs of unknown function. Deletion of SCD5 has shown that it is essential for cell growth at 30°C. scd5-Δ strains carrying low copy plasmids encoding C- terminal truncations of Scd5p are temperature sensitive for growth at 37°C. At the nonpermissive temperature, cells expressing a 338-amino acid deletion (Scd5p-Δ338) accumulate an internal pool of fully glycosylated invertase and mature α-factor, while processing and sorting of the vacuolar hydrolase carboxypeptidase Y is normal. The truncation mutant also accumulates 80- to 100-nm vesicles similar to many late sec mutants. Moreover, at 34°C, overexpression of Scd5p suppresses the temperature sensitivity of a sec2 mutant, which is blocked at a post-Golgi step of the secretory pathway. Biochemical analyses indicate that ~50% of Scd5p sediments with a 100,000 x g membrane fraction and is associated as a peripheral membrane protein. Overall, these results indicate that Scd5p is involved in vesicular transport at a late stage of the secretory pathway. Furthermore, this suggests that the lethality of clathrin-deficient yeast can be rescued by modulation of vesicular transport at this late secretory step.