The Malaria Parasite Cyclin H Homolog PfCyc1 Is Required for Efficient Cytokinesis in Blood-Stage Plasmodium falciparum

Abstract

All well-studied eukaryotic cell cycles are driven by cyclins, which activate cyclin-dependent kinases (CDKs), and these protein kinase complexes are viable drug targets. The regulatory control of the Plasmodium falciparum cell division cycle remains poorly understood, and the roles of the various CDKs and cyclins remain unclear. The P. falciparum genome contains multiple CDKs, but surprisingly, it does not contain any sequence-identifiable G1-, S-, or M-phase cyclins. We demonstrate that P. falciparum Cyc1 (PfCyc1) complements a G1 cyclin-depleted Saccharomyces cerevisiae strain and confirm that other identified malaria parasite cyclins do not complement this strain. PfCyc1, which has the highest sequence similarity to the conserved cyclin H, cannot complement a temperature-sensitive yeast cyclin H mutant. Coimmunoprecipitation of PfCyc1 from P. falciparum parasites identifies PfMAT1 and PfMRK as specific interaction partners and does not identify PfPK5 or other CDKs. We then generate an endogenous conditional allele of PfCyc1 in blood-stage P. falciparum using a destabilization domain (DD) approach and find that PfCyc1 is essential for blood-stage proliferation. PfCyc1 knockdown does not impede nuclear division, but it prevents proper cytokinesis. Thus, we demonstrate that PfCyc1 has a functional divergence from bioinformatic predictions, suggesting that the malaria parasite cell division cycle has evolved to use evolutionarily conserved proteins in functionally novel ways.

A divergent cyclin/cyclin-dependent kinase complex controls the atypical replication of a malaria parasite during gametogony and transmission

“…The observed absence of cycling may also reflect the constraints imposed by the extremely rapid nature of gametogony. No marked Cyc1 oscillations were detected during the slower P. falciparum blood stage schizogony and the Cyc1/MRK complex was detected in mature segmented schizonts after Cyc1 is required to complete cytokinesis 29 . Therefore, it is also possible that non-oscillating cell-cycle cyclins and stable cyclin/CDK complexes are a conserved feature of the various cycles of division within Plasmodium parasites.…”

“…Plasmodium genomes contain no sequenceidentifiable G1-, S-, or M-phase cyclins, and only three proteins have sequence homology with cyclin family members in other eukaryotes 10,11 . Cyc1 is important for cytokinesis in P. falciparum blood stage replication, possibly regulating the CDK7 homolog MRK 12 . P. berghei Cyc3 is dispensable for blood-stage replication but important for oocyst maturation in the mosquito midgut 11 .…”

Checks and balances? DNA replication and the cell cycle in Plasmodium

“…Transcriptional regulator? PF3D7_1014400; PBANKA_1212800 [ 38 , 44 , 46 ] PK6 Erythrocytic schizogony (trophozoite) Onset of S phase (cyclin independent) PF3D7_1337100; PBANKA_1350900 [ 47 ] Crks Crk-1 Gametocytes ( Pf ) Erythrocytic schizogony ( Pb ) Transcriptional regulator PF3D7_0417800; PBANKA_0719900 [ 114 , 115 ] Crk-3 Erythrocytic schizogony ( Pf ) Transcriptional regulator PF3D7_0415300; PBANKA_0717300 [ 42 ] Crk-5 Erythrocytic schizogony ( Pf ) Proliferation - number of merozoites. Activated by Cyc 1 and 4 in vitro.…”

“…Although PK5 is the putative homologue of mammalian CDK1, Plasmodium encodes no cognate cyclins for such an enzyme and the activator for PK5 remains unknown: in vitro , it is unusually promiscuous and can be activated by all three Plasmodium cyclins as well as mammalian p25, cyclin H and RINGO [ 37 , 44 , 45 ]. The partnership with Cyc1 is questionable because recent immunoprecipitation studies failed to identify it [ 46 ]. Nevertheless, PK5 has been shown to be involved in ORC1 phosphorylation, implicating it in DNA replication in erythrocytic stages [ 45 ].…”

In silico Screening and Evaluation of Plasmodium falciparum Protein Kinase 5 (PK5) Inhibitors

“…Of these, P. falciparum protein kinase 5 ( Pf PK5), which has been crystallized at 1.9 Å resolution, is a putative cyclin‐dependent kinase (CDK)‐like protein with the highest similarity in sequence and structure to Homo sapiens CDK2 ( Hs CDK2, 63 % sequence identity and 74 % similarity). Like Hs CDK2, Pf PK5 may regulate cell division, however this role has not been established in the parasite . The high degree of homology to Hs CDK2 (Figure S1) makes studying Pf PK5 function with small molecules during infection difficult and all known Pf PK5 inhibitors exhibit poor selectivity.…”