Ribulose-l,5-bisphosphate Carboxylase-Binding Chloroplast Thylakoid Membrane Proteins. In vitro Evidence that H +-ATP Synthase may Serve as a Membrane Receptor

Affinity binding to Sepharose 4 B-immobilized ribulose-1,5-bisphosphate carboxylase (Rubisco) and its isolated small (SSU) and large (LSU) subunits of Nonidet P40-solubilized chloroplast thylakoid proteins has been performed to search for Rubisco membrane receptors in vitro. It was found that Rubisco and both of its isolated subunit types specifically bind the same proteins including H +-ATP synthase, cytochrome f/b^ complex, ferredoxin-NADP+ re­ ductase (FNR) and components of photosystem (PS) II which presumably form multienzyme complexes. PS I was not recognized by Rubisco components. Of the Rubisco-binding thyla­ koid proteins, at least coupling factor 1 (CF,) is shown to form specific complexes with the enzyme as revealed by affinity chromatography and protein hybridization on nitrocellulose filters. Employing the latter approach, the SSU and LSU of Rubisco were observed to bind independently to isolated a and ß, but not to y, 5 and s subunits of CF,. The results indicate that H+-ATP synthase may serve as a Rubisco membrane receptor at least in vitro and suggest that the irreversible inhibition of photophosphorylation by Rubisco [R. T. Furbank et al., Biochim. Biophys. Acta 852,46-54 (1986)] is mainly due to its binding to the a/ß subunit pairs of CF,.