Insights into Positive and Negative Requirements for Protein-Protein Interactions by Crystallographic Analysis of the beta-Lactamase Inhibitory Proteins BLIP, BLIP-1, and BLP

Abstract

beta-Lactamase inhibitory protein (BLIP) binds a variety of beta-lactamase enzymes with wide-ranging specificity. Its binding mechanism and interface interactions are a well-established model system for the characterization of protein-protein interactions. Published studies have examined the binding of BLIP to diverse target beta-lactamases (e.g., TEM-1, SME-1, and SHV-1). However, apart from point mutations of amino acid residues, variability oil the inhibitor side of this enzyme-inhibitor interface has remained Unexplored. Thus, we present crystal structures of two likely BLIP relatives: (1) BLIP-T (solved alone and in complex with TEM-1), which has beta-lactamase inhibitory activity very similar to that of BLIP; and (2) beta-lactamase-inhibitory-protein-like protein (BLP) (in two apo forms, including all ultrahigh-resolution structure), which is unable to inhibit any tested beta-lactamase. Despite categorical differences in species of origin and function, BLIP-1 and BLP share nearly identical backbone conformations, even at loop regions differing in BLIP. We describe interacting residues and provide a comparative Structural analysis of the interactions formed at the interface of BLIP-1-TEM-1 versus those formed at the interface of BLIP-TEM-1. Along with initial attempts to functionally characterize BLP, we examine its amino acid residues that structurally correspond to BLIP/BLIP-I binding hotspots to explain its inability to bind and inhibit TEM-1. We conclude that the BLIP family fold is a robust and flexible scaffold that permits the formation of high-affinity protein-protein interactions while remaining highly selective. Comparison of the two naturally Occurring, distinct binding interfaces built upon this scaffold (BLIP and BLIP-I) shows that there is Substantial variation possible in the subnanomolar binding interaction with TEM-1. The corresponding (non-TEM-1-binding) BLP surface shows that numerous favorable backbone-backbone/backbone-side-chain interactions with a protein partner can be negated by the presence of a few, strongly unfavorable interactions, especially electrostatic repulsions. (C) 2009 Elsevier Ltd. All rights reserved.