Yeast protein translocation complex: Isolation of two genes SEB1 and SEB2 encoding proteins homologous to the Sec61 β subunit

A yeast gene (cDNA clone) was isolated in a screen for suppressors of secretion-defective sec15-1 mutation. This gene encodes a protein homologous to the β subunit of the mammalian Sec61 protein complex functioning in protein translocation into the endoplasmic reticulum (ER). The predicted protein, Seb1p, consists of 82 amino acids and contains one potential membrane-spanning region at the C-terminus but no N-terminal signal sequence. Seb1p shows 30% identity to the mammalian Sec61β subunit and 34% identity to the Arabidopsis thaliana Sec61β subunit. Overexpression of SEBI from a multicopy plasmid suppressed the temperature sensitivity of sec61-2 and sec61-3 mutants. Immunofluorescence and immunoelectron microscopy indicated that Seb1p resides in the ER membranes with the hydrophilic N-terminus exposed to the cytoplasm. The in vitro translated Seb1p was post-translationally inserted into microsomal membranes. As the chromosomal disruption of the SEB1 gene was not lethal, potential homologous genes were screened by heterologous hybridization. The SEB1 homologue thus isolated, SEB2, encodes a protein 53% identical to Seb1p. Disruption of the chromosomal SEB2 was not lethal whereas the double disruption of SEB1 and SEB2 resulted in a temperature-sensitive phenotype. This study further emphasizes the evolutionary conservation of the ER protein translocation apparatus and provides novel genetic tools for its functional analysis. The sequences of SEB1 and SEB2 have been deposited in the EMBL database under Accession Numbers Z47789 and Z50012. respectively. The sequence of the Seb1 protein is identical to that of Sbhlp independently identified by Panzner et al. (Cell 81, 561-570. 1995) using a biochemical approach.