STRUCTURE OF THE CARBOXY-TERMINAL FRAGMENT OF THE APO-BIOTIN CARBOXYLCARRIER SUBUNIT OF ESCHERICHIA-COLI ACETYL-COA CARBOXYLASE

The biotin carboxyl carrier protein (BCCP) is a subunit of acetyl-CoA carboxylase, a biotin-dependent enzyme that catalyzes the first commit ted step of fatty acid biosynthesis. In its functional cycle the bioti n carboxyl carrier protein engages in heterologous protein-protein int eractions with three distinct partners, depending on its state of post translational modification, Apo-BCCP interacts specifically with the b iotin holoenzyme synthetase, BirA, which results in the posttranslatio nal attachment of biotin to an essential lysine residue on BCCP, Holo- BCCP then interacts with the biotin carboxylase subunit, which leads t o the addition of the carboxylate group of bicarbonate to biotin. Fina lly, the carboxybiotinylated form of BCCP interacts with transcarboxyl ase in the conversion of acetyl-CoA to malonyl-CoA. The determinants o f protein-protein interaction specificity in this system are unknown, One hypothesis is that posttranslational modification of BCCP may resu lt in conformational changes that regulate specific protein-protein in teractions, To test this hypothesis, we have determined the NMR soluti on structure of the unbiotinylated form of an 87 residue C-terminal do main fragment of BCCP (apoBCCP87) from Escherichia coli acetyl-CoA car boxylase and compared this structure with the high-resolution structur e of the biotinylated form that was recently solved by X-ray crystallo graphic techniques. Although the overall folding of the two proteins i s highly similar, small structural differences are apparent for residu es of the biotin-binding loop that may be important for mediating spec ific protein-protein interactions.