Archaeosortase

An archaeosortase is a protein that occurs in the cell membranes of some archaea.^[1] Archaeosortases recognize and remove carboxyl-terminal protein sorting signals about 25 amino acids long from secreted proteins. A genome that encodes one archaeosortase may encode over fifty target proteins. The best characterized archaeosortase target is the Haloferax volcanii S-layer glycoprotein, an extensively modified protein with O-linked glycosylations, N-linked glycosylations, and a large prenyl-derived lipid modification toward the C-terminus.^[2] Recent work shows that either of two mutations, knockout of the archaeosortase A (artA) gene or permutation of the motif Pro-Gly-Phe (PGF) to Pro-Phe-Gly, blocks attachment of the lipid moiety as well as blocking removal of the PGF-CTERM protein-sorting domain.^[3] Thus archaeosortase appears to be a transpeptidase, like sortase, rather than a simple protease.

Archaeosortases are related to exosortases, their uncharacterized counterparts in Gram-negative bacteria. The names of both families of proteins reflect roles analogous to sortases in Gram-positive bacteria, with which they share no sequence homology. The sequences of archaeosortases and exosortases consists mostly of hydrophobic transmembrane helices, which sortases lack. Archaeosortases fall into a number of distinct subtypes, each responsible for recognizing sorting signals with a different signature motif. Archaeosortase A (ArtA) recognizes the PGF-CTERM signal, ArtB recognizes VPXXXP-CTERM, AtrC recognizes PEF-CTERM, and so on; one archaeal genome may encode two different archaeosortase systems.

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