Sec24p and Iss1p function interchangeably in transport vesicle formation from the endoplasmic reticulum in Saccharomyces cerevisiae

The Sec23p/Sec24p complex functions as a component of the COPII coat in ves icle transport from the endoplasmic reticulum. Here we characterize Sacchar omyces cerevisiae SEC24, which encodes a protein of 926 amino acids (YIL109 C), and a close homologue, ISS1 (YNL049C), which is 55% identical to SEC24. SEC24 is essential for vesicular transport in vivo because depletion of Se c24p is lethal, causing exaggeration of the endoplasmic reticulum and a blo ck in the maturation of carboxypeptidase Y. Overproduction of Sec24p suppre ssed the temperature sensitivity of sec23-2, and overproduction of both Sec 24p and Sec23p suppressed the temperature sensitivity of sec16-2. SEC24 gen e disruption could be complemented by overexpression of ISS1, indicating fu nctional redundancy between the two homologous proteins. Deletion of ISS1 h ad no significant effect on growth or secretion; however, iss1 Delta mutant s were found to be synthetically lethal with mutations in the v-SNARE genes SEC22 and BET1. Moreover, overexpression of ISS1 could suppress mutations in SEC22. These genetic interactions suggest that Iss1p may be specialized for the packaging or the function of COPII v-SNAREs. Iss1p tagged with His( 6) at its C terminus copurified with Sec23p. Pure Sec23p/Iss1p could replac e Sec23p/ Sec24p in the packaging of a soluble cargo molecule (alpha-factor ) and v-SNAREs (Sec22p and Bet1p) into COPII vesicles. Abundant proteins in the purified vesicles produced with Sec23p/Iss1p were indistinguishable fr om those in the regular COPII vesicles produced with Sec23p/Sec24p.