Characterization of Plasmodium falciparum Atypical Kinase PfPK7- Dependent Phosphoproteome.

PfPK7 is an "orphan" kinase displaying regions of homology to multiple protein kinase families. PfPK7 functions in regulating parasite proliferation/development as evident from the phenotype analysis of knockout parasites. Despite this regulatory role, the functions of PfPK7 in signaling pathways are not known. To better understand PfPK7-regulated phosphorylation events, we performed isobaric tag-based quantitative comparative phosphoproteomics of the schizont and segmenter stages from wild-type and pfpk7 - parasite lines. This analysis identified 3,875 phosphorylation sites on 1,047 proteins. Among these phosphorylation events, 146 proteins with 239 phosphorylation sites displayed reduction in phosphorylation in the absence of PfPK7. Further analysis of the phosphopeptides revealed three motifs whose phosphorylation was down regulated in the pfpk7 - cell line in both schizonts and segmenters. Decreased phosphorylation following loss of PfPK7 indicates that these proteins may function as direct substrates of PfPK7. We demonstrated that PfPK7 is active toward three of these potential novel substrates; however, PfPK7 did not phosphorylate many of the other proteins, suggesting that decreased phosphorylation in these proteins is an indirect effect. Our phosphoproteomics analysis is the first study to identify direct substrates of PfPK7 and reveals potential downstream or compensatory signaling pathways.

Global analysis of protein expression and phosphorylation of three stages of Plasmodium falciparum intraerythrocytic development.

During asexual intraerythrocytic development, Plasmodium falciparum diverges from the paradigm of the eukaryotic cell cycles by undergoing multiple rounds of DNA replication and nuclear division without cytokinesis. A better understanding of the molecular switches that coordinate a myriad of events for the progression of the parasite through the intraerythrocytic developmental stages will be of fundamental importance for rational design of intervention strategies. To achieve this goal, we performed isobaric tag-based quantitative proteomics and phosphoproteomics analyses of three developmental stages in the Plasmodium asexual cycle and identified 2767 proteins, 1337 phosphoproteins, and 6293 phosphorylation sites. Approximately 34% of identified proteins and 75% of phosphorylation sites exhibit changes in abundance as the intraerythrocytic cycle progresses. Our study identified 43 distinct phosphorylation motifs and a range of potential MAPK/CDK substrates. Further analysis of phosphorylated kinases identified 30 protein kinases with 126 phosphorylation sites within the kinase domain or in N- or C-terminal tails. Many of these phosphorylations are likely CK2-mediated. We define the constitutive and regulated expression of the Plasmodium proteome during the intraerythrocytic developmental cycle, offering an insight into the dynamics of phosphorylation during asexual cycle progression. Our system-wide comprehensive analysis is a major step toward defining kinase-substrate pairs operative in various signaling networks in the parasite.