Molecular Characterization, Assembly and Membrane Association of the Groel-Type Chaperonins in Synechocystis PCC 6803

Abstract

Chaperonins, a subclass of molecular chaperones, are a highly conserved family of proteins that are present in most, and probably all, living organisms. The level of their synthesis can be dramatically enhanced in response to stress, especially to heat stress (Georgopoulos, 1992). The GroE proteins of Escherichia coli are the best characterized molecular chaperones, at both the genetical and biochemical level, and have been used extensively to investigate interactions between this class of proteins and a number of target polypeptides (Lubben et al., 1989). There are two basic types of chaperonins of quite distinct sizes: the larger type known as GroEL contains subunits with an appropriate molecular mass of 60 kDa that are arranged in a complex structure comprising two rings of seven subunits stacked on each other. The second type of chaperonin (or co-chaperonin, GroES) is significantly smaller and contains subunits of about 10 kDa and, at least in bacteria, is thought to form a single ring of seven subunits. GroES and GroEL genes are essential for bacterial survival and they constitute an operon whose expression is enhanced during heat shock. It now appears that molecular chaperones exert their influence by stabilizing protein folding intermediates, thus partitioning them towards a pathway leading to the native state rather than forming inactive aggregated structures. As an implication of such effects, one of their main cellular functions is probably to provide protection under stress conditions (Gatenby et al., 1993).