Molecular analysis of an outer membrane protein, MopB, of Methylococcus capsulatus (Bath) and structural comparisons with proteins of the OmpA family

The gene encoding a major outer membrane protein (MopB) of the methanotroph Methylococcus capsulatus (Bath) was cloned and sequenced. The cloned DNA c ontained an open reading frame of 1044 bp coding for a 348-amino-acid polyp eptide with a 21-amino-acid leader peptide. Comparative sequence analysis o f the predicted amino acid sequence revealed that the C-terminal part of Mo pB possessed sequences that are conserved in the OmpA family of proteins. T he N-terminal half of the protein had no significant sequence similarity to other proteins in the databases, but the predicted secondary structure sho wed stretches of amphipathic beta-strands typical of transmembrane segments of outer membrane proteins. A region with four cysteines similar to the cy steine-encompassing region of the OprF of Pseudomonas aeruginosa was found toward the C-terminal part of MopB. Results from whole-cell labeling with t he fluorescent thiol-reacting reagent 5-iodoacetamidofluorescein indicated a surface-exposed location for these cysteines. A probe consisting of the 3 '-end of the mopB gene hybridized to the type I methanotroph Methylomonas m ethanica S1 in Southern blots containing DNA from nine methanotrophic strai ns representing six different genera.