The C-terminal part of the surface-associated protein MopE of the methanotroph Methylococcus capsulatus (Bath) is secreted into the growth medium

A protein with an apparent molecular mass of 46 kDa was detected as the maj or polypeptide in the culture medium of the biotechnologically important me thanotrophic bacterium Methylococcus capsulatus (Bath). The protein cross-r eacted with polyclonal antibodies raised against the outer-membrane-associa ted protein MopE. The antiserum was used to identify a positive clone from a lambda gt11 library. The nucleotide sequence determined for the clone dem onstrated that MopE and the secreted protein are encoded by the same gene, and that the secreted protein represents an N-terminally truncated form of MopE. By using monospecific antibodies against MopE in immunogold electron microscopy, the protein was localized at the cell surface and cell peripher y. The mopE gene was expressed in Escherichia coli. The MopE protein synthe sized was found in the periplasmic space of E. coli. No protein with sequen ce similarity over the entire length of MopE was detected in the databases, but some sequence similarity to the copper-repressible CorA protein of the methanotroph Methylomicrobium albus (Berson and Lidstrom 1997) was observe d for the C-terminal region of MopE.