The Staphylococcus aureus secretome - TI Pharma

Characterization of Staphylococcus aureus secretion mutants
DsbA does not seem to require a membrane-embedded partner protein for its re-oxidation
during catalysis. Instead, this protein is re-oxidized by components in the extracellular
environment (Heras et al., 2008;Kouwen et al., 2007).
In addition to the Sec pathway, Gram-positive bacteria can use several other special purpose
pathways for protein export from the cytoplasm. These include the YidC pathway, the Twinarginine
translocation (Tat) pathway, the Com pathway, secretion via holins or ABC
transporters, and the Ess pathway (Sibbald et al., 2006;Driessen and Nouwen, 2008;Yuan et
al., 2009).
YidC functions as a membrane insertase to mediate membrane protein insertion either by
itself or in concert with SecYEG. In the Sec pathway, YidC is linked to the Sec translocase
via SecDF (see (Yuan et al., 2009)). The YidC homologues in B. subtilis have been
designated SpoIIIJ and YqjG (Murakami et al., 2002;Tjalsma et al., 2003;Saller et al., 2009).
The Tat translocase consists of the TatA and TatC subunits. This translocase recognizes a
twin-arginine motif that consists of two adjacent (twin) Arg residues followed by another
amino acid and two hydrophobic amino acids (R-R-x-φ-φ, where φ is a hydrophobic amino
acid) (Cristóbal et al., 1999;Jongbloed et al., 2000). In contrast to the Sec pathway, this
translocase is able to transport folded proteins across the membrane, which seems of
particular relevance for the export of proteins with bound co-factors. The Tat pathway has
been studied very well in E. coli and B. subtilis (Dilks et al., 2003;Berks et al.,
2005;Robinson and Bolhuis, 2001), but fairly little is known about the roles of this pathway in
S. aureus. Recently, the Tat pathway of S. aureus has been studied using biochemical and
proteomics approaches, but this has led to the identification of only one substrate so far,
which is FepB (Yamada et al., 2007;Schmaler et al., 2009).
The Com pathway of B. subtilis is used for the export and assembly of pseudopili that are
necessary for DNA binding and uptake during natural genetic competence development
(Tjalsma et al., 2004;Tjalsma et al., 2000). Homologues of some of the components and the
secreted proteins are also present in S. aureus. So far, limited information is available about
this pathway in S. aureus. Morikawa and colleagues have shown that the expression of several
competence genes is under control of the SigH factor (Morikawa et al., 2003), but whether
these genes are involved in DNA uptake has so far remained unclear.
For S. aureus it has been shown that at least three proteins (i.e. EsxA, EsxB and EsaC) are
secreted via the ESX-1 or ESAT-6 secretion system (Ess) pathway (Burts et al., 2005;Burts et
al., 2008). First discovered in Mycobacterium tuberculosis, this pathway seems to be
conserved in several Gram-positive pathogens (Pallen, 2002). The EssA, EssB, and EssC
components are required for a functional Ess secretion machinery. Notably, proteins secreted
via the Ess pathway do not contain a classical signal peptide, but comparison of many of these
secreted proteins revealed a conserved WXG motif in the middle of these proteins (Pallen,
2002). Whether this motif represents an Ess targeting signal remains to be elucidated.
S. aureus and related Gram-positive bacteria have several mechanisms for the retention of
proteins that have been exported from the cytoplasm. One of these mechanisms is to bind the
protein to the membrane via a lipid anchor. Such lipoproteins are synthesized with a Sec type
signal peptide containing the so-called lipobox (von Heijne, 1989). The lipobox contains an
invariant Cys residue at the +1 position and the consensus sequence -3 L-x-x-C +1 . The lipobox
is recognized by the diacylglyceryl transferase Lgt and the invariant Cys is then modified by
transferring a diacylglyceryl group from phosphatidylglycerol to the sulfhydryl group of this
Cys residue (Tjalsma et al., 2000). Upon cleavage by the lipoprotein-specific signal peptidase
II, the lipid-modified Cys residue serves as the membrane anchor for the mature lipoprotein. It
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