Abstract
To prevent protein aggregation in cells, molecular chaperones stabilize non-native\r conformations of substrate (“client”) proteins and facilitate their correct folding. Clients often interact with chaperones in a defined order, but the mechanisms that enable coordinated interactions of clients with chaperones are poorly understood. One such case is the Hsp70 and Hsp90 families of chaperones in which clients are transferred from Hsp70 to Hsp90. This study focuses on the Hsp70/Hsp90 paralogs in the endoplasmic reticulum (BiP/Grp94) and uncovers the role of a BiP co-chaperone (ERdj3) in facilitating the sequential interactions of client proteins first with BiP, and then with Grp94.\r \r In the first part of my thesis, I show that the Erdj3 J-domain (JD) selectively binds the\r ATP conformation of BiP and subsequently drives BiP into the ADP conformation by\r stimulating BiP ATP hydrolysis. I develop a quantitative model that explains the influence of the JD on BiP ATPase activity and conformation. In the second part, I show that the JD/BiP interaction mechanism enables a well-defined sequence of chaperone interactions with BiP/JD coming first and Grp94/BiP coming second. Finally, I discover that phosphate release from BiP after ATP hydrolysis plays an important role for conformation of BiP and also for the coupling of BiP to Grp94.