Furthermore, Target Selective Inhibitor Library in a labeling experiment with the membrane-impermeable probe Mal-PEG, the ScFtsY N-terminal region was protected by the membrane and was not labeled. This observation indicates that this region was inserted into the membrane. Inner membrane proteins in bacteria are recognized during translation by the universally conserved signal recognition particle (SRP) and its receptor (SR). The bacterial SR, FtsY, is homologous to the SR-α subunit of the eukaryotic SR. The SR-α subunit is tethered to the membrane of the endoplasmic reticulum by its interaction

with the membrane-bound SR-β subunit (Gilmore et al., 1982; Angelini et al., 2006). However, no bacterial gene encoding an SR-β homolog has been identified in any bacterial genomes to date (Chater, 2006). The mechanisms by which bacterial FtsY interacts with the cytoplasmic membrane hence attracted much interest. The majority of the previous studies on FtsY membrane interaction have used Escherichia coli as a model system. The association of E. coli FtsY (EcFtsY) with the membrane involves two distinct

mechanisms (Angelini et al., 2006). EcFtsY can bind to the membrane through a protein–protein interaction. A direct learn more interaction between FtsY and a SecYEG translocon was observed (Angelini et al., 2005). A molecular modeling study suggested that the FtsY-Ffh complex can approach the SecYEG translocon with its G domains. FtsY can then be bound by the SecYEG translocon, specifically the cytoplasmic

loop of SecG and the C5/C6 loops of SecY (Chen et al., 2008). On the other hand, although EcFtsY is a highly charged protein without any predicted membrane-spanning segments, it is capable of directly targeting the membrane. There may be two lipid-binding domains that mediate this protein–lipid interaction (de Leeuw et al., 2000). One lipid-binding domain is located at the very N-terminus of EcFtsY (Weiche et al., 2008). The other lipid-binding domain is at the junction between the A domain and the conserved N domain, forming an amphipathic helix (Parlitz et al., 2007). Both of these two lipid-binding domains this website are not inserted into the membrane and locate close to the membrane surface (Braig et al., 2009). Compared to Gram-negative bacteria, little is known about how FtsY binds the membrane in Gram-positive bacteria. FtsY has three domains known as A/N/G (in the N-terminus to C-terminus orientation). The N and G domains are highly conserved. It is expected that the FtsY-SecYEG interaction mediated by the N/G domain will also be conserved in Gram-positive bacteria. Conversely, the FtsY A domain varies between species. In Bacillus subtilis, the A domain consists of only eight residues (Zanen et al., 2004), and FtsY is reported to appear soluble in vegetative cells (Rubio et al., 2005).

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