FMN is covalently attached to a threonine residue in the NqrB and NqrC subunits of Na+-translocating NADH-quinone reductase from Vibrio alginolyticus

The Na+-translocating NADH-quinone reductase (NQR) from Vibrio alginolyticu s is composed of six subunits (NqrA to NqrF). We previously demonstrated th at both NqrB and NqrC subunits contain a flavin cofactor covalently attache d to a threonine residue. Fluorescent peptide fragments derived from the Nq rB and NqrC subunits mere applied to a matrix-assisted laser desorption ion ization time-of-flight mass spectrometer, and covalently attached flavin wa s identified as FMN in both subunits, From post-source decay fragmentation analysis, it was concluded that FMN is attached by a phosphate group to Thr -235 in the NqrB subunit and to Thr-223 in the NqrC subunit, The phosphoest er binding of FMN to a threonine residue reported here is a new type of fla vin attachment to a polypeptide. (C) 2001 Federation of European Biochemica l Societies. Published by Elsevier Science B.V. All rights reserved.