The identification of a heat-shock protein complex in chloroplasts of barley leaves

In vivo radiolabeling of chloroplast proteins in barley (Hordeum vulgare L. cv Corvette) leaves and their separation by one-dimensional electrophoresis revealed at least seven heat-shock proteins between 24 and 94 kD, of which most have not been previously identified in this C(3) species. Fractionation into stromal and thylakoid membrane components showed that all chloroplast heat-shock proteins were synthesized on cytoplasmic ribosomes, translocated into the chloroplast, and located in the stroma. Examination of stromal preparations by native (nondissociating) polyacrylamide gel electrophoresis revealed the presence of a high-molecular mass heat-shock protein complex in barley. This complex was estimated to be 250 to 265 kD in size. Dissociation by denaturing polyacrylamide gel electrophoresis revealed a single protein component, a 32-kD heat-shock protein. The synthesis of this protein and the formation of the heat-shock protein complex were dependent on functional cytoplasmic ribosomes. Immunological studies showed that the heat-shock protein complex did not contain any proteins homologous to the alpha-subunit of ribulose bisphosphate carboxylase oxygenase subunit-binding protein. Other features about the complex included the absence of nucleic acid (RNA or DNA) and its nondissociation in the presence of Mg(2+)/ATP. These results suggest that the heat-shock protein complex in barley chloroplasts is a homogeneous octamer of 32-kD subunits.