Abstract
In C4 plants, the pyruvate phosphate dikinase regulatory protein (PDRP) regulates the C4 pathway enzyme pyruvate phosphate dikinase (PPDK) in response to changes in incident light intensity. In maize (Zea mays) leaves, two distinct isoforms of PDRP are expressed, ZmPDRP1 and ZmPDRP2. The properties and C4 function of the ZmPDRP1 isoform are well understood. However, the PDRP2 isoform has only recently been identified and its properties and function(s) in maize leaves are unknown. We therefore initiated an investigation into the maize PDRP2 isoform by performing a side by side comparison of its enzyme properties and cell-specific distribution with PDRP1. In terms of enzyme functionality, PDRP2 was found to possess the same protein kinase-specific activity as PDRP1. However, the PDRP2 isoform was found to lack the phosphotransferase activity of the bifunctional PDRP1 isoform except when PDRP2 in the assays is elevated 5- to 10-fold. A primarily immuno-based approach was used to show that PDRP1 is strictly expressed in mesophyll cells and PDRP2 is strictly expressed in bundle sheath strand cells (BSCs). Additionally, using in situ immunolocalization, we establish a regulatory target for PDRP2 by showing a significant presence of C4 PPDK in BSC chloroplasts. However, a metabolic role for PPDK in this compartment is obscure, assuming PPDK accumulating in this compartment would be irreversibly inactivated each dark cycle by a monofunctional PDRP2.
Highlights
In the C4 metabolic pathway, the pyruvate, phosphate dikinase (PPDK) regulatory protein (PDRP) regulates PPDK activity according to the level of incident light (Chastain, 2011)
PDRP2 as a possible regulator of PPDK in bundle sheath strand cells (BSCs) chloroplasts expression of the C4 PPDK transcript may have contributed to the selection of a BSC-specific phosphate dikinase regulatory protein (PDRP) isoform to regulate PPDK enzyme that accumulates in this compartment
We extend this observation by providing evidence to show that the expressed polypeptides of PDRP2 and PDRP1 are BSC and mesophyll cell (MC) specific, respectively
Summary
In the C4 metabolic pathway, the pyruvate, phosphate dikinase (PPDK) regulatory protein (PDRP) regulates PPDK activity according to the level of incident light (Chastain, 2011). PDRP regulates PPDK in a strict on/off fashion via its ability to phosphorylate reversibly a threonine residue (Thr456 in maize) within the PPDK active site (Fig. 1). When this residue is phosphorylated, PPDK is inactive, and vice versa. PDRP was assumed to be encoded by a single gene, ZmPDRP1 (GRMZM2G131286), corresponding to the first reported PDRP gene cloned in 2006 (Burnell and Chastain, 2006). With the release of the higher resolution version-2 maize B73 genome, a second PDRP gene, ZmPDRP2 (GRMZM2G004880), was revealed, along with transcript evidence of its expression
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