The role of phosphorylation and dephosphorylation in the regulation of Rubisco activase

Rubisco activase (Rca) is an ATP-dependent chaperone that facilitates dissociation of inhibitory sugar phosphates from the catalytic sites of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) during photosynthesis. In Arabidopsis, Rca is negatively regulated by dark-dependent phosphorylation of Thr78. Here we identify chloroplast localized SHEWENELLA-LIKE PROTEIN PHOSPHATASE 1 (SLP1) as the major phosphatase responsible for light-dependent Rca dephosphorylation. The prevalence of Thr78 in Rca was investigated across sequences from 91 plant species, finding 29 (∼32%) species shared a threonine in the same position. Analysis of seven C3 species with an antibody raised against a Thr78 phospho-peptide demonstrated that this position is phosphorylated in multiple genera. However, light-dependent dephosphorylation of Thr78 was observed only in Arabidopsis. Further, phosphorylation of Thr78 could not be detected in any of the four C4 grass species examined. The results suggest that despite conservation of Thr78 in Rca from a wide range of species, a regulatory role for phosphorylation at this site may be limited. This provides a case study for how variation in post-translational regulation can amplify functional divergence across the phylogeny of plants beyond what is explained by sequence variation in a metabolically important protein.