Gail K. Donaldson

Several proteins imported into chloroplasts form stable complexes with the GroEL-related chloroplast molecular chaperone.

Nine different proteins were imported into isolated pea chloroplasts in vitro. For seven of these [the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), beta-subunit of ATP synthase, glutamine synthetase, the light-harvesting chlorophyll a/b binding protein, chloramphenicol acetyltransferase, and pre-beta-lactamase], a fraction was found to migrate as a stable high-molecular-weight complex during nondenaturing gel electrophoresis. This complex contained the mature forms of the imported proteins and the groEL-related chloroplast chaperonin 60 (previously known as Rubisco subunit binding protein). Thus, the stable association of imported proteins with this molecular chaperone is widespread and not necessarily restricted to Rubisco subunits or to chloroplast proteins. With two of the imported proteins (ferredoxin and superoxide dismutase), such complexes were not observed. It seems likely that, in addition to its proposed role in assembly of Rubisco, the chloroplast chaperonin 60 is involved in the assembly or folding of a wide range of proteins in chloroplasts.

The Cellular Functions of Chaperonins

It is apparent that a major sub-set of heat shock proteins assist other polypeptides to maintain, or assume, a conformation required for their correct assembly into biologically active structures (Georgopoulos et al. 1973; Kochan and Murialdo 1983; Goloubinoff et al. 1989; Cheng et al. 1989; Ostermann et al. 1989; Bresnick et al. 1989) or localization (Deshaies et al. 1988; Chirico et al. 1988; Zimmermann et al. 1988; Bochkareva et al. 1988; Lecker et al. 1989). This group of proteins function as molecular chaperones, and they have been defined as proteins which assist the assembly of some oligomeric proteins, but are not components of the final structure (Ellis 1987; Ellis et al. 1989; Ellis and Hemmingsen 1989). One distinct group of related molecular chaperones are found in prokaryotes, mitochondria, and plastids, and are called chaperonins (Hemmingsen et al. 1988). In this chapter we outline the discovery and characterization of chaperonins in prokaryotic and eukaryotic organisms, and also describe recent data that show that these proteins have an important role in protein folding in cells.