A Newly Uncovered Ppdk Regulatory Protein Isoform In Maize Catalyzes Regulatory Phosphorylation But Not Dephosphorylation Of Ppdk

The PPDK regulatory protein (PDRP) catalyzes the reversible phosphorylation of a Thr residue located within the central catalytic active‐site of pyruvate phosphate dikinase (PPDK). In higher plants, PPDK is an integral enzyme of the C4 photosynthetic pathway and therefore its concomitant up‐/down phospho‐regulation by PDRP is believed to be critical for optimizing photosynthetic efficiency. PDRP is a member of the bacterial and plant DUF‐299 family of proteins now recognized from 1° structure as either PDRP or PEP synthetase regulatory protein (PSRP). The catalytic properties of PDRP (and PSRP) are singularly unique among protein phosphorylating/dephosphorylating enzymes. Among these are: (i) bifunctionality, catalyzing both PPDK phosphorylation and dephosphorylation, (ii) use of ADP versus ATP as a phosphoryl donor, and (iii) utilization of an Pi‐dependent, PPi‐forming phosphorolytic dephosphorylation mechanism. Notably, the PDRP 1° structure is devoid of any canonical subdomains or motifs that unifies all known eukaryotic and prokaryotic Ser/Thr protein kinases into one of three superfamilies. Recently, we identified a second PDRP isoform in maize (PDRP2) that lacks the phospho‐PPDK dephosphorylating activity of its bifunctional counterpart, PDRP1. Alignment of the highly similar PDRP1 and PDRP2 primary sequence does not reveal any discernable residue differences that could account for the lack of dephosphorylating activity of PDRP2 vs. PDRP1. We conclude that (3‐D) structural features not evident in the primary structures of PDRP1 and PDRP2 likely accounts for the stark contrast in catalytic properties of two isoforms.