Yet1p and Yet3p,the Yeast Homologs of BAP29 and BAP31, Interact with the Endoplasmic Reticulum Translocation Apparatus and Are Required for Inositol Prototrophy

The mammalian B-cell receptor-associated proteins of 29 and 31 kDa (BAP29 and BAP31) are conserved integral membrane proteins that have reported roles in endoplasmic reticulum (ER) quality control, ER export of secretory cargo, and programmed cell death. In this study we investigated the yeast homologs of BAP29 and BAP31, known as Yet1p and Yet3p, to gain insight on cellular function. We found that Yet1p forms a complex with Yet3p (Yet complex) and that complex assembly was important for subunit stability and proper ER localization. The Yet complex was not efficiently packaged into ER-derived COPII vesicles and therefore does not appear to act as an ER export receptor. Instead, a fraction of the Yet complex was detected in association with the ER translocation apparatus (Sec complex). Specific mutations in the Sec complex or Yet complex influenced these interactions. Moreover, associations between the Yet complex and Sec complex were increased by ER stress and diminished when protein translocation substrates were depleted. Surprisingly, yet1Δ and yet3Δ mutant strains displayed inositol starvation-related growth defects. In accord with the biochemical data, these growth defects were exacerbated by a combination of certain mutations in the Sec complex with yet1Δ or yet3Δ mutations. We propose a model for the Yet-Sec complex interaction that places Yet1p and Yet3p at the translocation pore to manage biogenesis of specific transmembrane secretory proteins.