Trigger factor associates with GroEL in vivo and promotes its binding to certain polypeptides

Trigger factor (TF) is a putative molecular chaperone recently found to function together with GroEL in the degradation of the fusion protein, CRAG. TF overproduction enhanced the ability of GroEL to form complexes with CRAG, as well as fetuin or histone. To define further this effect on GroEL binding, affinity columns containing a variety of denatured proteins were used. When cell extracts were applied onto a fetuin column, both TF and GroEL bound but not GroES. Upon ATP addition, TF and GroEL were eluted together and remained tightly associated (even in presence of GroES) in complexes containing one TF per GroEL 14-mer. Overproduction of TF enhanced the capacity of GroEL to bind to many denatured proteins. Moreover, GroEL-TF complexes isolated from such cells showed much greater binding capacity than GroEL from TF-deficient cells. Furthermore, the addition of pure TF to pure GroEL also enhanced markedly its binding capacity. The affinity of GroEL for CRAG also rises during heat shock due to GroEL phosphorylation. TF expression, however, did not promote GroEL phosphorylation. Moreover, heat shock and TF overproduction affected GroEL binding to other denatured polypeptides in distinct ways; only TF promoted binding to certain polypeptides, whereas only phosphorylation increased binding to others. Thus, association with TF and phosphorylation are independent regulators of GroEL function. This enhanced affinity of TF-GroEL complexes for unfolded proteins may also be important in protein folding, because TF has prolyl isomerase activity and associates with nascent polypeptides.