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

β-Lactamase inhibitory protein (BLIP) binds a variety of β-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 β-lactamases (e.g., TEM-1, SME-1, and SHV-1). However, apart from point mutations of amino acid residues, variability on the inhibitor side of this enzyme–inhibitor interface has remained unexplored. Thus, we present crystal structures of two likely BLIP relatives: (1) BLIP-I (solved alone and in complex with TEM-1), which has β-lactamase inhibitory activity very similar to that of BLIP; and (2) β-lactamase-inhibitory-protein-like protein (BLP) (in two apo forms, including an ultra-high-resolution structure), which is unable to inhibit any tested β-lactamase. Despite categorical differences in species of origin and function, BLIP-I and BLP share nearly identical backbone conformations, even at loop regions differing in BLIP.