Identification of residues in beta-lactamase critical for binding beta-lactamase inhibitory protein.

beta-Lactamase inhibitory protein (BLIP) is a potent inhibitor of several beta-lactamases including TEM-1 beta-lactamase (Ki = 0.1 nM). The co-crystal structure of TEM-1 beta-lactamase and BLIP has been solved, revealing the contact residues involved in the interface between the enzyme and inhibitor. To determine which residues in TEM-1 beta-lactamase are critical for binding BLIP, the method of monovalent phage display was employed. Random mutants of TEM-1 beta-lactamase in the 99-114 loop-helix and 235-240 B3 beta-strand regions were displayed as fusion proteins on the surface of the M13 bacteriophage. Functional mutants were selected based on the ability to bind BLIP. After three rounds of enrichment, the sequences of a collection of functional beta-lactamase mutants revealed a consensus sequence for the binding of BLIP. Seven loop-helix residues including Asp-101, Leu-102, Val-103, Ser-106, Pro-107, Thr-109, and His-112 and three B3 beta-strand residues including Ser-235, Gly-236, and Gly-238 were found to be critical for tight binding of BLIP. In addition, the selected beta-lactamase mutants A113L/T114R and E240K were found to increase binding of BLIP by over 6- and 11-fold, respectively. Combining these substitutions resulted in 550-fold tighter binding between the enzyme and BLIP with a Ki of 0.40 pM. These results reveal that the binding between TEM-1 beta-lactamase and BLIP can be improved and that there are a large number of sequences consistent with tight binding between BLIP and beta-lactamase.