Exosortase
Exosortase refers to a family of integral membrane proteins that occur in Gram-negative bacteria that recognizes and cleaves the carboxyl-terminal sorting signal PEP-CTERM. The name derives from a predicted role analogous to sortase, despite the lack of any detectable sequence homology, and a strong association of exosortase genes with exopolysaccharide or extracellular polymeric substance biosynthesis loci. Many archaea have an archaeosortase, homologous to exosortases rather than to sortases. Archaeosortase A recognizes the signal PGF-CTERM, found at the C-terminus of some archaeal S-layer proteins. Following processing by archaeosortase A, the PGF-CTERM region is gone, and a prenyl-derived lipid anchor is present at the C-terminus instead.
Exosortase has not itself been characterized biochemically. However, site-directed mutagenesis work on archaeosortase A, an archaeal homolog of exosortases, strongly supports the notion of a Cys active site and convergent evolution with sortase family transpeptidases. In 2018, a study on Zoogloea resiniphila, a bacterium found in activated sludge wastewater treatment plants, showed that PEP-CTERM proteins are essential to floc formation in some systems. More recently, Parrett, et al. showed that cyanoexosortase B, the lone exosortase in the filamentous Cyanobacterium Nostoc punctiforme, is required both for a colony gliding behavior and for completion of the biosynthesis of the UV-absorbing pigment scytonemin. Both processes depend on PEP-CTERM proteins.