ELECTROSPRAY MASS-SPECTROMETRIC INVESTIGATION OF THE CHAPERONE SECB

Electrospray ionization mass spectrometry was used to investigate the structure of the Escherichia coli chaperone protein SecB. It was deter mined that the N-terminal methionine of SecB has been removed and that more than half of all SecB monomers are additionally modified, most l ikely by acetylation of the N-terminus or a lysine. The use of gentle mass spectrometer interface conditions showed that the predominant, ol igomeric form of SecB is a tetramer that is stable over a range of sol ution pH conditions and mass spectrometer interface heating (i.e., inl et capillary temperatures). At very high pH, SecB dimers are observed. SecB contains a region that is hypersensitive to cleavage by proteina se K and is thought to be involved in conformational changes that are crucial to the function of SecB. We identified the primary site of cle avage to be between Leu 141 and Gin 142. Fourteen amino acids are remo ved, but the truncated form remains a tetramer with stability similar to that of the intact form.