SYMMETRICAL COMPLEXES OF GROE CHAPERONINS AS PART OF THE FUNCTIONAL CYCLE

The particular structural arrangement of chaperonins probably contribu tes to their ability to assist in the folding of proteins. The interac tion of the oligomeric bacterial chaperonin GroEL and its cochaperonin , GroES, in the presence of adenosine diphosphate (ADP) forms an asymm etric complex. However, in the presence of adenosine triphosphate (ATP ) or its nonhydrolyzable analogs, symmetric complexes were found by el ectron microscopy and image analysis. The existence of symmetric chape ronin complexes is not predicted by current models of the functional c ycle for GroE-mediated protein folding. Because complete folding of a nonnative substrate protein in the presence of GroEL and GroES only oc curs in the presence of ATP, but not with ADP, the symmetric chaperoni n complexes formed during the GroE cycle are proposed to be functional ly significant.