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Environmental and Stress Physiology

The Identification of a Heat-Shock Protein Complex in Chloroplasts of Barley Leaves ¹

Department of Botany, The University of Queensland, QLD 4072, Australia

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 C₃ 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 -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 Mg²⁺/ATP. These results suggest that the heat-shock protein complex in barley chloroplasts is a homogeneous octamer of 32-kD subunits.

¹ A.K.C. was supported in part by a University of Queensland Postgraduate Research Scholarship.

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