Two proton pump interactors identified from a direct phosphorylation screening of a rice cDNA library by using a recombinant BRI1 receptor kinase

A direct phosphorylation screening of a rice cDNA library resulted in isolation of 35 BIP clones encoding brassinosteroid receptor kinase (BRI1)-interacting proteins. Among the candidate substrates for BRI1, two clones were found to encode similar proton pump interactor proteins homologous to Arabidopsis PPI1, which was reported to interact with a regulatory region of plasma membrane H⁺-ATPase. The rice proton pump interactors BIP103 and BIP131 contained 627 and 621 amino acids, respectively, with carboxyl-terminal hydrophobic region characteristic of tail-anchored proteins. Northern blotting analysis indicated that mRNAs for both interactors increased significantly after brassinolide treatment of lamina joint cells, which are especially sensitive to exogenous brassinosteroids.