Prolific release of S-layer units and associated proteins by the methanotrophic bacterium Methylomicrobium album BG8

Some gammaproteobacterial methanotrophs are encapsulated by S-layers that overlay their outer membrane. In this study, TEM imaging revealed that Methylomicrobium album BG8 constitutively releases abundant cup-shaped S-layer units - a phenotype that was not observed in 7 other methanotroph strains, even in Methylotuvimicrobium buryatense 5GB1 that produces a similar S-layer structure. Release of S-layer units occurred regardless of carbon (methane or methanol) or nitrogen (ammonium or nitrate) source, with 50X trace metal availability or under copper deprivation, and at all growth phases. The released S-layer units were isolated from the culture medium of M. album BG8 for proteome analysis. The proteome revealed an abundance of transporters related to calcium uptake including TolC and Repeats-in-Toxin (RTX) proteins, transporters for uptake for cobalamin and siderophores, proteins with Type I secretion system (T1SS) target domains, cell wall biogenesis proteins, and the S- layer protein units themselves. The gene encoding the S-layer protein is co-located in the M. album BG8 genome with genes encoding the T1SS. Together, this information provides clues regarding M. album BG8 S-layer biogenesis and function, including a role for the calcium and RTX protein domains, involvement of the T1SS in selecting associated proteins, and reasons behind prolific release of S-layer units. This S-layer release and involvement of the T1SS has biological implications and could be applied towards selective secretion of proteins by M. album BG8 in bioindustrial applications. ImportanceThe methanotrophic bacterium M. album BG8 produces and releases large quantities of its S-layer units into the culture medium regardless of carbon or nitrogen source, metal availability or phase of growth. Of the 8 methanotrophic bacteria screened, only M. album BG8 possesses this phenotype. Proteomics analysis of purified S-layer units identified the S-layer protein itself along with associated proteins involved metal uptake and S-layer biogenesis, including involvement of the T1SS. Better understanding of M. album BG8 S-layer biogenesis and the release of its S-layer units into culture medium could be harnessed for exporting expressed proteins of bioindustrial interest for ease of collection and purification.