ATP binding and ATP hydrolysis in full-length MsbA monitored via time-resolved Fourier transform infrared spectroscopy

The essential Escherichia coli ATPase MsbA is a lipid flippase that serves as a prototype for multi drug resistant ABC transporters. Its physiological function is the transport of lipopolisaccharides to build up the outer membranes of gram negative bacteria. Although several structural and biochemical studies of MsbA have been conducted previously, a detailed picture of the dynamic processes that link ATP hydrolysis to allocrit transport remains elusive. We report here for the first time time-resolved Fourier transform infrared (FTIR) spectroscopic measurements of the ATP binding and ATP hydrolysis reaction of full-length MsbA and determined reaction rates at 288 K of k(1) = 0.49 +/- 0.28 s(-1) and k (2) = 0.014 +/- 0.003 s(-1), respectively. We further verified these rates with photocaged NPEcgAppNHp where only nucleotide binding was observable and the negative mutant MsbA-H537A that showed slow hydrolysis (k(2) < 2 x 10(-4) s(-1)). Besides single turnover kinetics, FTIR measurements also deliver IR signatures of all educts, products and the protein. ADP remains protein-bound after ATP hydrolysis. In addition, the spectral changes observed for the two variants MsbA-S378A and MsbA-S482A correlated with the loss of hydrogen bonding to the gamma-phosphate of ATP. This study paves the way for FTIR-spectroscopic investigations of allocrite transport in full-length MsbA.