Trigger factor is involved in GroEL-dependent protein degradation in Escherichia coli and promotes binding of GroEL to unfolded proteins.

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In Escherichia coli, the molecular chaperones of hsp60/hsp10 (GroEL/GroES) families are required not only for protein folding but also for the rapid degradation of certain abnormal proteins. The rate-limiting step in the degradation of the fusion protein CRAG by protease ClpP appears to be the formation of a complex with GroEL. We have isolated these complexes and found that each GroEL 14mer contained a short-lived fragment of CRAG plus a 50 kDa polypeptide, which we identified by sequencing and immunological methods as Trigger Factor (TF). Upon ATP addition, GroEL and TF dissociated together from CRAG but remained tightly associated with each other even upon gel filtration. TF was originally proposed to function in protein translocation across membranes but altering cellular content of TF did not affect this process in vivo. By contrast, low levels of TF expression markedly reduced CRAG degradation, while an overproduction of TF greatly stimulated this process. Furthermore, in extracts of cells expressing high levels of TF, the capacity of GroEL to bind to CRAG is greatly increased. Overproduction of TF also stimulated GroEL's ability to bind to other unfolded proteins (fetuin and histone). Thus, TF is a rate-limiting factor for CRAG degradation; it appears to regulate GroEL function and to promote the formation of TF-GroEL-CRAG complexes which are critical for proteolysis.

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