TRIGGER FACTOR IS INVOLVED IN GROEL-DEPENDENT PROTEIN-DEGRADATION IN ESCHERICHIA-COLI AND PROMOTES BINDING OF GROEL TO UNFOLDED PROTEINS

In Escherichia call, the molecular chaperones of hsp60/hsp10 (GroEL/Gr oES) families are required not only for protein folding but also for t he rapid degradation of certain abnormal proteins. The rate-limiting s tep in the degradation of the fusion protein CRAG by protease ClpP app ears to be the formation of a complex with GroEL. We have isolated the se complexes and found that each GroEL 14mer contained a short-lived f ragment of CRAG plus a 50 kDa polypeptide, which we identified by sequ encing and immunological methods as Trigger Factor (TF), Upon ATP addi tion, GroEL and TF dissociated together from CRAG but remained tightly associated with each other even upon gel filtration. TF was originall y proposed to function in protein translocation across membranes but a ltering cellular content of TF did not affect this process in vivo. By contrast, low levels of TF expression markedly reduced CRAG degradati on, while an overproduction of TF greatly stimulated this process. Fur thermore, in extracts of cells expressing high levels of TF, the capac ity of GroEL to bind to CRAG is greatly increased. Overproduction of T F also stimulated GroEL's ability to bind to other unfolded proteins ( fetuin and histone), Thus, TF is a rate-limiting factor for CRAG degra dation; it appears to regulate GroEL function and to promote the forma tion of TF-GroEL-CRAG complexes which are critical for proteolysis.