Imported large subunits of ribulose bisphosphate carboxylase/oxygenase, but not imported β-ATP synthase subunits, are assembled into holoenzyme in isolated chloroplasts

Abstract

The large subunit of ribulose bisphosphate carboxylase from Anacystis nidulans 6301, and the beta subunit of chloroplast ATP synthase from maize, were fused to the transit peptide of the small subunit of ribulose bisphosphate carboxylase from soybean. These proteins were assayed for post-translational import into isolated pea chloroplasts. Both proteins were imported into chloroplasts. Imported large subunits were associated with two distinct macromolecular structures. The smaller of these structures was a hybrid ribulose bisphosphate carboxylase holoenzyme, and the larger was the binding protein oligomer. Time-course experiments following import of the large subunit revealed that the amount of large subunit associated with the binding protein oligomer decreased over time, and that the amount of large subunit present in the assembled holoenzyme increased. We also observed that imported small subunits of ribulose bisphosphate carboxylase, although predominantly present in the holoenzyme, were also found associated with the binding protein oligomer. In contrast, the imported beta subunit of chloroplast ATP synthase did not assemble into a thylakoid-bound coupling factor complex.