Cotranslational activity of the GroEL chaperonin

Various proteins in the cell begin to fold during synthesis at the ribosome, many with the assistance of molecular chaperones. While the cotranslational activity of ribosome-associated chaperones and Hsp70 is frequently studied, the role of Hsp60 chaperonins during protein synthesis remains poorly understood. Here, we studied the binding of E. coli chaperonin GroEL to ribosome-nascent chain complexes (RNCs), to understand GroEL activity during nascent chain (NC) synthesis and folding. Using biochemical reconstitution, structural proteomics and electron microscopy we describe the physical architecture of GroEL:RNC and ATP/BeFx-GroEL/ES:RNC complexes. We show that GroEL engages destabilised nascent chains on the inside of its cavity via the apical domains and disordered C-terminal tails. This GroEL:RNC complex can be capped by GroES on the NC-bound ring, with GroEL but not GroEL/ES binding promoting nascent chain destabilisation. Lastly, we observe that GroEL directly competes with Hsp70 for nascent chain binding. Our findings thus show that GroEL is a versatile chaperone which in addition to its well characterised post-translational activity can affect folding of nascent chains undergoing synthesis.