Conformational plasticity of a BiP-GRP94 chaperone complex

Hsp70/Hsp90-chaperones and their regulatory co-chaperones are critical for maintaining protein homeostasis. GRP94, the sole Hsp90-chaperone in the secretory pathway of mammalian cells, is essential for the maturation of important secretory and transmembrane proteins. Without the requirement of co-chaperones, the Hsp70-protein BiP controls regulatory conformational changes of GRP94 - the structural basis of which has remained elusive. Here, we biochemically and structurally characterize the formation of a BiP-GRP94 chaperone complex and its transition to a conformation expected to support the loading of substrate proteins from BiP onto GRP94. BiP initially binds to the open GRP94 dimer via an interaction interface that is conserved among Hsp70/90 paralogs. Subsequently, binding of a second BiP protein stabilizes a semi-closed GRP94 dimer, thereby advancing the chaperone cycle. Our findings highlight a fundamental mechanism of direct Hsp70/90 cooperation, independent of co-chaperones.