Monday, 01 January, 2018
Bap (Sil1) regulates the molecular chaperone BiP by coupling release of nucleotide and substrate
M. Rosam, D. Krader, C. Nickels, J. Hochmair, K. C. Back, G. Agam, A. Barth, C. Zeymer, J. Hendrix, M. Schneider, I. Antes, J. Reinstein, D. C. Lamb and J. Buchner -
Nature Structural & Molecular Biology, 10.1038/s41594-017-0012-6 (2018)
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.