Reversible phosphorylation of photosynthetic PEP carboxylase: Studies on C4-leaf PP2A and recombinant PEPC-kinase from CAM-induced Mesembryanthemum crystallinum

Abstract

The activity and allosteric properties of plant PEPC are controlled posttranslationally by specific reversible phosphorylation of a strictly conserved Ser residue near the N-terminus. This up/down-modulation is catalyzed by a transcriptionally regulated, seemingly dedicated Ser/Thr kinase (PpcK) and an opposing Ser/Thr phosphatase (PP2A). We have now partially purified and characterized the native form of this largely "neglected" PP2A from maize leaves using various chromatographic and affinity matrices, and C4 [32P]PEPC as substrate (Dong et al., 2001, Planta [in press]). The results indicate that the C4-leaf holoenzyme is analogous to yeast and mammalian PP2As in regards to its heterotrimeric structure (~170 kDa), comprised of a ~103-kDa core PP2Ac-A heterodimer complexed with a ~74-kDa B-type subunit, and its sensitivity to free Me2+ and various inhibitors, activators and anionic metabolites. Notably, this native PP2A (a) lacks any strict phosphoprotein specificity in that it dephosphorylates C4 PEPC, mammalian phosphorylase a, and casein in vitro, and (b) displays, at best, only modest light/dark effects in vivo on its apparent Mr, component core subunits, and activity against C4 PEPC-SerP. In addition, we will also report new findings on a recombinant form of CAM PpcK from M. crystallinum (Taybi et al., 2000, Plant Physiol.) produced as a highly soluble, active fusion with the ~55-kDa NusA carrier protein in E. coli. This NusA¿PpcK fusion protein has been purified by sequential IMAC and FPLC, used for detailed analysis of its target-protein specificity and other kinetic properties, and cleaved "on-bead" by thrombin to yield free PpcK for antibody production.