COOPERATION OF ENZYMATIC AND CHAPERONE FUNCTIONS OF TRIGGER FACTOR INTHE CATALYSIS OF PROTEIN-FOLDING

The trigger factor of Escherichia coli is a prolyl isomerase and accel erates proline-limited steps in protein folding with a very high effic iency. It associates with nascent polypeptide chains at the ribosome a nd is thought to catalyse the folding of newly synthesized proteins. I n its enzymatic mechanism the trigger factor follows the Michaelis-Men ten equation. The unusually high folding activity of the trigger facto r originates from its tight binding to the folding protein substrate, as reflected in the low K-m value of 0.7 mu M. In contrast, the cataly tic constant k(cat) is small and shows a value of 1.3 s(-1) at 15 degr ees C. An unfolded protein inhibits the trigger factor in a competitiv e fashion. The isolated catalytic domain of the trigger factor retains the full prolyl isomerase activity towards short peptides, but in a p rotein folding reaction its activity is 800-fold reduced and no longer inhibited by an unfolded protein. Unlike the prolyl isomerase site, t he polypeptide binding site obviously extends beyond the FKBP domain. Together, this suggests that the good substrate binding, i.e. the chap erone property, of the intact trigger factor is responsible for its hi gh efficiency as a catalyst of proline-limited protein folding.