Structurally Related Spc1p and Spc2p of Yeast Signal Peptidase Complex Are Functionally Distinct

Abstract

Two subunits of the mammalian signal peptidase complex, SPC12 and SPC25, share similar membrane topologies with the majority of each protein oriented toward the cytoplasm. Such similarities may suggest that these proteins perform redundant functions in signal peptidase activity. In the present study, we addressed this issue through analysis of the yeast homologs to SPC12 and SPC25, Spc1p and Spc2p. We show that both Spc1p and Spc2p are nonessential for signal peptidase activity and growth of yeast cells and that null mutations in the genes encoding Spc1p and Spc2p are synthetically lethal with a conditional mutation affecting Sec11p, an essential subunit of yeast signal peptidase. However, a high copy plasmid encoding Spc1p suppresses the conditional sec11 mutation, whereas the corresponding plasmid encoding Spc2p does not suppress sec11. Moreover, Spc2p, but not Spc1p, is important for signal peptidase activity and cell viability at high temperatures. These results indicate that although both Spc1p and Spc2p are noncatalytic, they are functionally distinct. Evidence is also presented that a double mutant lacking Spc1p and Spc2p grows well relative to wild type yeast cells, indicating that the signal peptidase complex missing at least two of its subunits is sufficient for signal peptidase activity in vivo.

In Addition to SEC11, a Newly Identified Gene,SPC3, Is Essential for Signal Peptidase Activity in the Yeast Endoplasmic Reticulum

“…2). These measurements are similar to the homologies shared between Spc1p and Spc2p and their mammalian counterparts (6,7). Spc3p is also related to the SPC22/23 homolog (18) present in the two-subunit avian SPC (54% similarity, 24% identity).…”

“…The spc3-4 Mutation Interacts in a Synthetic Manner with sec11-7 and ⌬spc2 but Not ⌬spc1-The sec11-7 mutation displays a genetic interaction, termed synthetic lethality, with the ⌬spc1 and ⌬spc2 mutations (6,7). Specifically, the growth of cells containing the sec11 mutation and either the spc1 or spc2 null mutation is inhibited more severely than the growth of cells containing these mutations individually.…”

“…Whole cell extracts were also prepared from control strain CMYD1 (sec11) (lane 19) following a 2-h incubation at 37°C. Proteins were resolved on an SDS-PAGE gel (7%) and subjected to Western blot analysis with anti-Kar2p antibodies (7). Whole cell extracts were prepared as described (4,28).…”

“…Spc2p is important for enzyme activity and cell viability at elevated temperatures (7). Spc1p and Spc2p are homologous to mammalian subunits SPC12 and SPC25, respectively (6,7,11,12).Despite the fact that a multisubunit signal peptidase has been purified from yeast and mammalian cells, enzymes exhibiting less subunit complexity have been identified in other systems. Leader peptidase from the inner membrane of E. coli consists of a single polypeptide chain (13).…”

“…Spc2p is important for enzyme activity and cell viability at elevated temperatures (7). Spc1p and Spc2p are homologous to mammalian subunits SPC12 and SPC25, respectively (6,7,11,12).…”