Sortase domain found in the class E family of sortases
Class E sortases are membrane-bound cysteine transpeptidases distributed in Gram-positive bacteria (mainly present in Actinobacteria). Genes encoding class A and E sortases are never found in the same organism, and similar to class A sortases, the genes encoding class E sortases are not positioned adjacent to genes encoding potential protein substrates, suggesting a housekeeping sortase function of class E sortases in some high G + C Gram-positive bacteria. Similar to class A sortase, class E sortases are capable of anchoring a large number of functionally distinct surface proteins containing a cell wall sorting signal to an amino group located on the bacterial cell wall. They recognize an LAXTG sorting signal, instead of the canonical LPXTG motif processed by class A sortases. The prototypical class E sortase from Corynebacterium diphtheria (named Cd-SrtF) is a non-polymerization sortase that is not required for pilus polymerization, and proceeds to complete the assembly process by anchoring the polymer to the cell wall peptidoglycan. Moreover, in Streptomyces coelicolor, one or both of Staphylococcus aureus SrtA homologs may function as class E sortase responsible for the cell wall anchoring of the long chaplin proteins (ChpA-C) containing an LAXTG sorting signal, which presumably mediate aerial hyphae formation. The family also includes some class E sortase homologs from Gram-negative and Archaebacterial species, but the functions of these enzymes are unknown.