The Sec23p/Sec24p complex functions as a component of the COPII coat in vesicle transport from the endoplasmic reticulum. Here we characterize Saccharomyces cerevisiae SEC24, which encodes a protein of 926 amino acids (YIL109C), and a close homologue, ISS1 (YNL049C), which is 55% identical to SEC24. SEC24 is essential for vesicular transport in vivo because depletion of Sec24p is lethal, causing exaggeration of the endoplasmic reticulum and a block in the maturation of carboxypeptidase Y. Overproduction of Sec24p suppressed the temperature sensitivity of sec23-2, and overproduction of both Sec24p and Sec23p suppressed the temperature sensitivity of sec16-2. SEC24 gene disruption could be complemented by overexpression ofISS1, indicating functional redundancy between the two homologous proteins. Deletion of ISS1 had no significant effect on growth or secretion; however, iss1Δ mutants were found to be synthetically lethal with mutations in the v-SNARE genes SEC22 and BET1. Moreover, overexpression of ISS1 could suppress mutations inSEC22. These genetic interactions suggest that Iss1p may be specialized for the packaging or the function of COPII v-SNAREs. Iss1p tagged with His₆ at its C terminus copurified with Sec23p. Pure Sec23p/Iss1p could replace Sec23p/Sec24p in the packaging of a soluble cargo molecule (α-factor) and v-SNAREs (Sec22p and Bet1p) into COPII vesicles. Abundant proteins in the purified vesicles produced with Sec23p/Iss1p were indistinguishable from those in the regular COPII vesicles produced with Sec23p/Sec24p.