PfSRPK1, a Novel Splicing-related Kinase from Plasmodium falciparum

Abstract

Even though it is increasingly evident that post-transcriptional events like mRNA processing and splicing may regulate gene expression and proteome diversity of malaria parasite Plasmodium, molecular mechanisms that regulate events like mRNA splicing in malaria parasite are poorly understood. Protein kinases control a wide variety of cellular events in almost all eukaryotes, including modulation of mRNA splicing, transport, and stability. We have identified a novel splicing-related protein kinase from Plasmodium falciparum, PfSRPK1. PfSRPK1 when incubated with parasite nuclear extracts inhibited RNA splicing, suggesting that it may control mRNA splicing in the parasite. PfSR1, a putative splicing factor from P. falciparum, was identified as a substrate of PfSRPK1. PfSR1 interacts with RNA and PfSRPK1 modulates its RNA binding. Early in the parasite development, PfSRPK1 and PfSR1 are present in the nucleus. These studies provide useful insights into the function of two potentially key components of P. falciparum mRNA splicing machinery.Malaria is one of the most serious infectious diseases and causes several million deaths and clinical illness in hundreds of millions of people every year (1). One of the major problems of recent times has been the emergence of new drugresistant strains of the malaria parasite Plasmodium falciparum. After invasion of the erythrocytes, the parasite can propagate asexually, giving rise to several new merozoites that invade fresh erythrocytes, which serve as hosts for the parasite to propagate. The blood stage infection is the cause of malaria pathogenesis. The parasite also undergoes sexual differentiation, which leads to the formation of male and female gametocytes. Upon ingestion of gametocytes by the Anopheles mosquito, further development continues inside the vector host. It is well known that signal transduction events mediated by protein kinases regulate diverse cellular processes. The importance of protein kinases in malaria parasite has been highlighted by a series of recent reports (2, 3), as these enzymes have been implicated in wide ranging events in both asexual and sexual stages of the parasite life cycle and, therefore, are considered as potential drug targets. Despite these reports, the information regarding signaling networks in the parasite is very limited.Although it is clear that metazoan protein kinase cascades control gene expression by regulating transcriptional or posttranscriptional events, this area in P. falciparum biology is largely unexplored. Plasmodium displays a high degree of developmental control of gene expression (4). Strikingly, only a few transcription factors have been identified in the parasite leading to the speculation that post-transcriptional events may be pivotal in regulating gene expression in the parasite (5). Translational repression of some genes (6) and control of gene expression by antisense RNA are some of the post-transcriptional (7) events that have been implicated in regulating gene expression in the parasite. mRNA splicing is on...

Culture adaptation of malaria parasites selects for convergent loss-of-function mutants

“…The gene product SRPK1 catalyzes phosphorylation of the parasite PfSR1 (Ser/Arg-rich) protein, involved in mRNA splicing in the nucleus23. The SRPK1 gene transcript is expressed in P. falciparum asexual stages and at an even higher level in gametocytes24, and its orthologue is necessary for male gamete maturation in rodent malaria parasites25.…”

The kinomes of apicomplexan parasites

“…Moreover, and as shown by reverse genetics, these two enzymes are necessary for completion of the parasite's asexual replication cycle. Dixit et al also showed that PfCLK4 (which they refer to as PfSRPK1) inhibits mRNA splicing by phosphorylating PfSR1, a putative splicing factor known to interact with RNA [82].…”

Transcript maturation in apicomplexan parasites

“…Apicomplexa parasites also encode this capacity with an exceptional expansion of the RRM-domain family (1% of total genes) that includes conserved SR and hnRNP proteins [29]. Despite the abundance of RNA-binding proteins and potential importance to parasite biology there has been limited research on these factors (TgDRE1 [23, 47]; PfSR1 [48, 49]). Alternative splicing is regulated by phosphorylation of the SR factors by cdc2-like kinases (CLK) of the CMGC family.…”

“…Alternative splicing is regulated by phosphorylation of the SR factors by cdc2-like kinases (CLK) of the CMGC family. Apicomplexa genomes encode four members of CLK kinase family [48, 50]. Given the homolog of PfCLK-1/Lammer and PfCLK-2 kinases to yeast splicing kinase Sky1p it was expected these kinases would have a role in alternate splicing.…”

“…The Plasmodium ortholog of Sky1p substrate Npl3p protein, splicing factor PfASF-1, was actively phosphorylated by these essential kinases [50], however, the link between regulation of the alternative splicing is yet to be established. Another member of the Plasmodium CLK family of kinases, PfSRPK1 not only phosphorylated splicing factor PfSF1 but showed inhibitory effect on RNA splicing in nuclear extracts [48]. …”