Kinetic partitioning between alternative protein-protein interactions controls a transcriptional switch

Proteins can perform completely distinct functions in response to the particular partners that they bind to. Consequently, determination of the mechanism of functional regulation in such systems requires elucidation of the mechanism switching between binding partners. The central protein of the Escherichia coli biotin regulatory system, BirA, switches between its function as a metabolic enzyme or a transcriptional repressor in response to binding either the biotin carboxyl carrier protein subunit of acetyl-CoA carboxylase or a second BirA monomer. These two protein-protein interactions are structurally mutually exclusive. The results of earlier studies suggest that the system is regulated by kinetic partitioning between the two protein-protein interactions. In this work, sedimentation velocity was employed to monitor the partitioning directly. The results indicate similar equilibrium parameters governing formation of the two protein-protein interactions. Kinetic analysis of the sedimentation velocity data indicated that holoBirA dimerization is governed by very slow forward and reverse rate constants. The slow kinetics of holoBirA dimerization combined with fluctuations in the intracellular apoBCCP pool are critical determinants in partitioning BirA between its distinct biological functions.