On 2014 Sep 14, Daniel Haft commented:

EpsH family proteins in bacteria are now called exosortase. Their very distant homologs in the archaea are termed archaeosortase. Exosortases and archaeosortases belong to an extended superfamily, exclusive to prokaryotes, whose members can be detected by TIGRFAMs model TIGR04178. An updated description of the superfamily occurs in PMID:22037399 (Haft, et al., 2012). The variety of archaeosortase and exosortase systems should recall the situation with sortases and their substrates, "An embarrassment of sortases - a richness of substrates?" (PMID:11239768, Pallen, et al. 2001).

The first characterized member of the extended family is archaeosortase A, ArtA, from Haloferax volcanii. Its primary target is the major cell surface glycoprotein, which forms the S-layer. This target has been a model for studying post-translational modification in the archaea. Knocking out ArtA blocks removal of the PGF-CTERM sorting signal and causes a variety of phenotypic differences include S-layer defects. See Abdul Halim, et al., 2013, PMID:23651326.

The work on archaeosortase suggests that both exosortases and archaeosortases may be transpeptidases. The S-layer glycoprotein was previously known to have a large prenyl-derived lipid, attached somewhere toward the C-terminus. Its purpose was unclear because the final C-terminal transmembrane helix seemed sufficient to anchor the protein to membrane. However, proof that the PGF-CTERM sorting signal is removed during maturation suggests that the lipid replaces it as the anchor. Transpeptidase activity would allow removal of the C-terminal sorting signal and attachment of the lipid to occur simultaneously.

On 2014 Sep 14, Daniel Haft commented:

EpsH family proteins in bacteria are now called exosortase. Their very distant homologs in the archaea are termed archaeosortase. Exosortases and archaeosortases belong to an extended superfamily, exclusive to prokaryotes, whose members can be detected by TIGRFAMs model TIGR04178. An updated description of the superfamily occurs in PMID:22037399 (Haft, et al., 2012). The variety of archaeosortase and exosortase systems should recall the situation with sortases and their substrates, "An embarrassment of sortases - a richness of substrates?" (PMID:11239768, Pallen, et al. 2001).

The first characterized member of the extended family is archaeosortase A, ArtA, from Haloferax volcanii. Its primary target is the major cell surface glycoprotein, which forms the S-layer. This target has been a model for studying post-translational modification in the archaea. Knocking out ArtA blocks removal of the PGF-CTERM sorting signal and causes a variety of phenotypic differences include S-layer defects. See Abdul Halim, et al., 2013, PMID:23651326.

The work on archaeosortase suggests that both exosortases and archaeosortases may be transpeptidases. The S-layer glycoprotein was previously known to have a large prenyl-derived lipid, attached somewhere toward the C-terminus. Its purpose was unclear because the final C-terminal transmembrane helix seemed sufficient to anchor the protein to membrane. However, proof that the PGF-CTERM sorting signal is removed during maturation suggests that the lipid replaces it as the anchor. Transpeptidase activity would allow removal of the C-terminal sorting signal and attachment of the lipid to occur simultaneously.