Signal peptide peptidase

Signal peptidases also Signalasen, are protein-splitting enzymes (proteases) that (ER) are located within the cell to the membrane of the endoplasmic reticulum. The signal peptidase recognizes specific amino acid sequences ( signal sequences ) within a protein. It splits in the lumen of the ER from the signal peptide from the rest of the polypeptide chain.

Structure

In mammals, the enzyme is composed of five membrane proteins ( see Fig.) The subunits SPC12, SPC18, SPC21, SPC25 SPC22/23 and were named after her in gel electrophoresis (SDS- PAGE) certain molecular mass in kilodaltons. Three proteins, SPC18, SPC21 SPC22/23 and each having only a transmembrane segment to be anchored in the membrane of the ER, where the N -terminus in the cytosol and the majority of the protein is located together with the C -terminus in the lumen of the ER. In SPC18 SPC21 and is almost identical proteins, they provide 80 % matching amino acids to each other isoforms dar. SPC22/23 is a glycoprotein which runs as a double band at 22 kDa and 23 kDa in SDS -PAGE. The mass of SPC22/23 without glycosylation is 19 kDa. The sub-units each have SPC12 SPC25 and two transmembrane segments, the C- and N -termini of the proteins are localized in the cytoplasm. Furthermore, the distance between the membrane anchors in both proteins is so low that possess SPC25 SPC12 and hardly luminal areas. SPC18 and SPC21 are probably constituents of the active site of the signal peptidase. Except in mammalian signal peptidase was also characterized in the bird and in yeast in more detail. Of chicken oviduct cells, a signal peptidase complex was purified, which consists only of the two membrane proteins, gp23 and p19. The gp23 is homologous to mammalian subunit SPC22/23 and p19 is homologous to SPC18.

Eukaryotic and prokaryotic signal peptidases

All signal peptidases that cleave the N -terminal signal peptides are summarized I signal peptidases in the group of type. These include, inter alia, bacterial leader peptidase, mitochondrial inner membrane and signal peptidases of the ER signal peptidases. The substrate specificity of the type I signal peptidases is highly conserved: prokaryotic leader peptidases can cleave signal sequences of eukaryotic transport substrates, as well as vice versa eukaryotic signal peptidases prokaryotic transport substrates litigate. In contrast to the eukaryotic signal peptidases, however, the prokaryotic leader peptidases consist of a membrane protein, the most well known is the leader peptidase LepB from E. coli, respectively. Sequence comparisons revealed that several regions in the yeast Sec11p as well as in the mammalian subunits have SPC21 SPC18 and homologies to bacterial Leaderpeptidase.

Active site

By point mutation studies of prokaryotic leader peptidases could be shown that a conserved serine residue and a lysine residue are essential for the cleavage activity. The crystal structure analysis of Leaderpeptidase from E. coli revealed that form the serine residue at position 90 with lysine at position 145 of the active site of the peptidase. In contrast to classical serine endopeptidases acting in the Leader peptidases probably lysine instead of histidine as a base in the cleavage reaction. Interestingly, in the eukaryotic ER signal peptidases, although the serine residue conserved, however, is the sequence comparison in the position of the lysine histidine. Point mutation studies in Sec11p also revealed that none of the protein occurring in the lysine is essential for the cleavage activity. As essential for cleavage, however, His- 83 were Ser - 44, Asp -103 and Asp -109 identified, which tends to favor a classical serine endopeptidase. Remarkably, however, in this context, that the ER signal peptidases may be inhibited by any of the classical serine protease inhibitors (such as aprotinin).

Importance

The cleavage of proteins by signal peptidases membrane- plays an important role in the post-translational processing and localization of proteins. This is particularly true for enveloped viruses that mature at the membrane of the ER and the polyprotein is cleaved by cellular signal peptidases in part, eg Members of the virus family Flaviviridae (BVDV, hepatitis C virus, yellow fever virus).

• Cell Biology