Duality monomer-dimer of the pheromone-binding protein from Bombyx mori

The analysis of a recombinant pheromone-binding protein from the silkworm m oth, Bombyx mori, by native gel electrophoresis with Coomassie staining sho wed one single band with a molecular mass consistent with a monomer. A slow migrating band, detected in the recombinant and native samples by a polycl onal antibody, was indistinguishable from the monomer in the mass spectrum fragmentation pattern and chromatographic behavior. Flow injection analyses of the protein by mass spectrometry in the negative mode showed fragments of a dimer. The dimeric form was also supported by estimation of the molecu lar mass by gel filtration at basic pH, A cross-linked dimer coeluted with the noncovalent dimer on a gel filtration column. The molecular mass of the protein changed in a pi-I-dependent way with a dramatic transition from di mer to monomer between pH 6 and 4.5. A low pH induced not only dissociation of the dimer, but also a conformational change in the protein. In marked c ontrast to denaturation with guanidinium chloride, the emission maxima of t ryptophan was not significantly changed at low pH, BmPBP is thus a dimer at slightly acid, neutral, and basic pH, which dissociates and then undergoes conformational change at low pH. (C) 2000 Academic Press.