Bok binds to a largely disordered loop in the coupling domain of type 1 inositol 1,4,5-trisphosphate receptor.

Bcl-2-related ovarian killer (Bok) binds tightly to inositol 1,4,5-trisphosphate receptors (IP3 Rs). To better understand this interaction, we sought to elucidate the Bok binding determinants in IP3 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 IP3 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 IP3 R1-derived peptides revealed high affinity binding to amino acids 1898-1940 of IP3 R1 (Kd ∼65 nM) and that binding affinity is also dependent upon both of the high disorder flanking regions. The strength of the Bok-IP3 R1 interaction was demonstrated by the ability of IP3 R1 or Bok to recruit transmembrane domain-free Bok or IP3 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 IP3 R1 occurs within a largely disordered loop between α helices 72 and 73 and that high affinity binding is mediated by multivalent interactions.