Signal peptidase

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

structure

Schematic representation of the ER signal peptidase complex from mammalian cells. In mammals, the complex consists of five membrane proteins. The transmembrane segments of the proteins are shown as gray boxes. SPC22 / 23 is available as a glycoprotein, which is symbolized by the attached circle.

In mammals , the enzyme is composed of five membrane proteins (see fig.). The subunits SPC12, SPC18, SPC21, SPC22 / 23 and SPC25 were each named after their molecular mass in kilodaltons as determined by gel electrophoresis ( SDS-PAGE ) . Three proteins, SPC18, SPC21 and SPC22 / 23 each have only one transmembrane segment for anchoring in the membrane of the ER, the N terminus being located in the cytosol and the majority of the protein together with the C terminus in the lumen of the ER. SPC18 and SPC21 are almost identical proteins, they are isoforms with 80% matching amino acids . SPC22 / 23 is a glycoprotein that 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 subunits SPC25 and SPC12 each have two transmembrane segments, the C and N termini of the proteins being located in the cytoplasm. Furthermore, the distance between the membrane anchors in both proteins is so small that SPC25 and SPC12 hardly have any luminal areas. SPC18 and SPC21 are believed to be components of the active site of signal peptidase. In addition to mammals, signal peptidase has also been characterized in more detail in birds and yeast. A signal peptidase complex consisting only of the two membrane proteins gp23 and p19 was purified from chicken fallopian tube cells. The gp23 is homologous to the mammalian subunit SPC22 / 23 and p19 is homologous to SPC18.

Eukaryotic and prokaryotic signal peptidases

All signal peptidases that split off N-terminal signal peptides are summarized in the group of type I signal peptidases . These include the bacterial leader peptidases , the mitochondrial signal peptidases of the inner membrane and the ER signal peptidases. The substrate specificity of type I signal peptidases is strongly conserved: prokaryotic leader peptidases can cleave signal sequences from eukaryotic transport substrates , just as, conversely, eukaryotic signal peptidases can cleave prokaryotic transport substrates . In contrast to the eukaryotic signal peptidases, however, the prokaryotic leader peptidases each consist of only one membrane protein, the best-known representative of which is the leader peptidase LepB from E. coli . Sequence comparisons revealed that several regions in yeast Sec11p and also in the mammalian subunits SPC21 and SPC18 show homologies to the bacterial leader peptidase.

Active center

Point mutation studies on prokaryotic leader peptidases have shown that a conserved serine residue and a lysine residue are essential for the cleavage activity. The crystal structure analysis of the leader peptidase from E. coli showed that the serine residue at position 90 together with lysine at position 145 form the active center of the peptidase. In contrast to classic serine endopeptidases , in the leader peptidases lysine probably acts as the base in the cleavage reaction instead of histidine . Interestingly, although the serine residue is conserved in the eukaryotic ER signal peptidases, there is a histidine in the position of the lysine when the sequence is compared. Point mutation studies on Sec11p also showed that none of the lysines found in the protein is essential for the cleavage activity. In contrast, Ser-44, His-83, Asp-103 and Asp-109 were identified as essential for the cleavage, which suggests a classic serine endopeptidase. However, it is noteworthy in this context that the ER signal peptidases cannot be inhibited by any of the classic serine protease inhibitors (e.g. aprotinin ).

meaning

The cleavage of proteins by membrane signal peptidases plays an important role in the post-translational processing and localization of proteins. This also applies in particular to enveloped viruses that mature on the ER membrane and whose polyprotein is partially cleaved by cellular signal peptidases, e.g. B. Members of the Flaviviridae virus family ( BVDV , hepatitis C virus , yellow fever virus ).