BiP is the endoplasmic member of the Hsp70 family. BiP is regulated by several co-chaperones including the nucleotide-exchange factor (NEF) Bap (Sil1 in yeast). Bap is a two-domain protein. The interaction of the Bap C-terminal domain with the BiP ATPase domain is sufficient for its weak NEF activity. However, stimulation of the BiP ATPase activity requires full-length Bap, suggesting a complex interplay of these two factors. Here, single-molecule FRET experiments with mammalian proteins reveal that Bap affects the conformation of both BiP domains, including the lid subdomain, which is important for substrate binding. The largely unstructured Bap N-terminal domain promotes the substrate release from BiP. Thus, Bap is a conformational regulator affecting both nucleotide and substrate interactions. The preferential interaction with BiP in its ADP state places Bap at a late stage of the chaperone cycle, in which it coordinates release of substrate and ADP, thereby resetting BiP for ATP and substrate binding.

BiP是Hsp70家族的内质成员。BiP受包括核苷酸交换因子（NEF）Bap（酵母中的Sil1）在内的几种辅助伴侣的调节。Bap是一个两个域的蛋白质。Bap C末端结构域与BiP ATPase结构域的相互作用足以弥补其弱NEF活性。但是，BiP ATPase活性的刺激需要全长Bap，提示这两个因素之间存在复杂的相互作用。在这里，用哺乳动物蛋白进行的单分子FRET实验表明，Bap影响两个BiP域的构象，包括盖子亚域，这对底物结合很重要。很大程度上未结构化的Bap N末端结构域促进了底物从BiP释放。因此，Bap是一种影响核苷酸和底物相互作用的构象调节剂。