The copper responding surfaceome of Methylococccus capsulatus Bath

Proteins on the cellular surface of a bacterium, its surfaceome, are part of the interface between the bacterium and its environment, and are essential for the cells response to its habitat. Methylococcus capsulatus Bath is one of the most extensively studied methane-oxidizers and is considered as a model-methanotroph. The composition of proteins of the surfaceome of M. capsulatus Bath varies with the availability of copper and changes significantly upon only minor changes of copper concentration in the sub-μM concentration range. Proteins that respond to the changes in copper availability include the assumed copper acquisition protein MopE, c-type heme proteins (SACCP, cytochrome c(553o) proteins) and several proteins of unknown function. The most intriguing observation is that multi-heme c-type cytochromes are major constituents of the M. capsulatus Bath surfaceome. This is not commonly observed in bacteria, but is a feature shared with the dissimilatory metal-reducing bacteria. Their presence on the M. capsulatus Bath cellular surface may be linked to the cells ability to efficiently adapt to changing growth conditions and environmental challenges. However, their possible role(s) in methane oxidation, nitrogen metabolism, copper acquisition, redox-reactions and/or electron transport remain(s) at present an open question. This review will discuss the possible significance of these findings.