capsulatus Dabrafenib Bath (Kao et al., 2004; Karlsen et al., 2005a). The extensive physiological changes in lifestyle were efficiently demonstrated with ICAT (isotope-coded affinity tag)-labelling of high- and low-copper grown cells combined with downstream LC-MS/MS, revealing more than 500 differentially expressed proteins (Kao et al., 2004). However, these cultures represented the extremes of copper concentrations in the growth medium, and much less is known regarding gene expression over the copper concentration

range where the switch in lifestyle actually takes place. Proteins of the outer membrane are part of the interface between the bacterium and its environment, and are essential for cells in their response to its habitat. These proteins must be diverse in function, including protection against environmental challenges, uptake of growth factors, and bacterial interaction (Navarre & Schneewind, 1999; Hancock & Brinkman, 2002; Borges-Walmsley et al., 2003; Odenbreit, 2005; Scott, 2006). We have recently described both the outer

membrane proteome (integral- and outer membrane associated proteins exposed to the periplasm or cell exterior) and the surfaceome (proteins associated to the cellular surface) of M. capsulatus Bath, and how the composition of the surfaceome significantly changes with only minor changes in the availability of copper during growth (Table 1) (Berven et al., 2003; Karlsen et al., 2008). In the following sections, we will review some of the findings on the M. capsulatus Bath surface-exposed proteins, their copper dependent expression, and the intriguing enrichment of c-type heme proteins on the cell surface. MopE was originally identified as one of five abundant proteins (MopA-E) present in the outer membrane of M. capsulatus Bath (Fjellbirkeland et al., 1997). The cellular localization of MopE was further determined by immunogold-conjugated 4-Aminobutyrate aminotransferase antibody labelling

and NaCl-extraction of whole cells, demonstrating that MopE is surface exposed and noncovalently associated to the cell surface (Fjellbirkeland et al., 2001; Karlsen et al., 2005b). Furthermore, an N-terminal truncated form of MopE (MopE*) is secreted in significant amounts to the growth medium (Fjellbirkeland et al., 2001). The exact mechanisms of the cellular translocation of MopE, and how the processing of MopE to MopE* occurs is still unknown. However, MopE is synthesized with an N-terminal leader sequence, indicating a sec-dependent translocation of the protein across the Gram-negative inner membrane. The expression of mopE is induced when copper becomes limited, starting before the copper-switch and is highest in sMMO-expressing cells (Karlsen et al., 2003). In sMMO-expressing cells, MopE is the most prominent protein in the M. capsulatus Bath surfaceome (Table 1) (Karlsen et al., 2008), and MopE* is now also abundant in the growth medium (Karlsen et al., 2003).

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