Abstract
Bcl-2-related ovarian killer (Bok) binds tightly to inositol 1,4,5-trisphosphate receptors (IP(3)Rs). To better understand this interaction, we sought to elucidate the Bok binding determinants in IP(3)R1, focusing on the ∼75 amino acid loop (residues 1882-1957) between α helices 72 and 73. Bioinformatic analysis revealed that the majority of this loop is intrinsically disordered, with two flanking regions of high disorder next to a low disorder central region (∼residues 1914-1926) that is predicted to contain two fused, disjointed transient helical elements. Experiments with IP(3)R1 mutants, combined with computational analysis, indicated that small deletions in this central region block Bok binding due to perturbation of the helical elements. Studies in vitro with purified Bok and IP(3)R1-derived peptides revealed high affinity binding to amino acids 1898-1940 of IP(3)R1 (K(d) ∼65 nM) and that binding affinity is also dependent upon both of the high disorder flanking regions. The strength of the Bok-IP(3)R1 interaction was demonstrated by the ability of IP(3)R1 or Bok to recruit transmembrane domain-free Bok or IP(3)R1 mutants, respectively, to membranes in intact cells, and that these two mutants can bind in the cytosol independently of membrane association. Overall, we show that Bok binding to IP(3)R1 occurs within a largely disordered loop between α helices 72 and 73 and that high affinity binding is mediated by multivalent interactions.