The Saccharomyces cerevisiae SOM1 gene: heterologous complementation studies, homologues in other organisms, and association of the gene product with the inner mitochondrial membrane.

The small nuclear gene SOM1 of Saccharomyces cerevisiae was isolated as a multicopy suppressor of a mutation in the IMP1 gene, which encodes the mitochondrial inner membrane peptidase subunit 1 (Imp1). Analysis revealed that Som1 and Imp1 are components of a mitochondrial protein export system, and interaction between these two proteins is indicated by the genetic suppression data. Here we describe the identification of a gene from Kluyveromyces lactis, which restores respiratory function to a S. cerevisiae SOM1 deletion mutant at 28 degrees C. The sequence of the K. lactis gene predicts a protein product of 8.1-kDa, comprising 71 amino acid residues, with a putative mitochondrial signal sequence at its N-terminus. The protein is 50% identical to its S. cerevisiae counterpart. The expression pattern of a homologous sequence in Leishmania major suggests a more general role for SOM1 in mitochondrial biogenesis and protein sorting. The various Som1 proteins exhibit a highly conserved region and a remarkable pattern of cysteine residues. A protein of the expected size was transcribed and translated in vitro. The Som1 protein was detected in fractions of S. cerevisiae enriched for mitochondria and found to be associated with the inner mitochondrial membrane.