title: Identification and initial characterization of three novel cyclin-related proteins of the human malaria parasite Plasmodium falciparum.
abstract: The molecular mechanisms regulating cell proliferation and development during the life cycle of malaria parasites remain to be elucidated. The peculiarities of the cell cycle organization during Plasmodium falciparum schizogony suggest that the modalities of cell cycle control in this organism may differ from those in other eukaryotes. Indeed, existing data concerning Plasmodium cell cycle regulators such as cyclin-dependent kinases reveal structural and functional properties that are divergent from those of their homologues in other systems. The work presented here lies in the context of the exploitation of the recently available P. falciparum genome sequence toward the characterization of putative cell cycle regulators. We describe the in silico identification of three open reading frames encoding proteins with maximal homology to various members of the cyclin family and demonstrate that the corresponding polypeptides are expressed in the erythrocytic stages of the infection. We present evidence that these proteins possess cyclin activity by demonstrating either their association with histone H1 kinase activity in parasite extracts or their ability to activate PfPK5, a P. falciparum cyclin-dependent kinase homologue, in vitro. Furthermore, we show that RINGO, a protein with no sequence homology to cyclins but that is nevertheless a strong activator of mammalian CDK1/2, is also a strong activator of PfPK5 in vitro. This raises the possibility that "cryptic" cell cycle regulators may be found among the 50% of the open reading frames in the P. falciparum genome that display no homology to any known proteins.
authors.affiliation: INSERM U511 team, Wellcome Centre for Molecular Parasitology, Anderson College, 56 Dumbarton Road, Glasgow G11 6NU, Scotland, United Kingdom.
authors.forename: Anaïs
authors.initials: A
authors.lastname: Merckx
authors.forename: Karine
authors.initials: K
authors.lastname: Le Roch
authors.forename: Marie-Paule
authors.initials: MP
authors.lastname: Nivez
authors.forename: Dominique
authors.initials: D
authors.lastname: Dorin
authors.forename: Pietro
authors.initials: P
authors.lastname: Alano
authors.forename: Gustavo J
authors.initials: GJ
authors.lastname: Gutierrez
authors.forename: Angel R
authors.initials: AR
authors.lastname: Nebreda
authors.forename: Dean
authors.initials: D
authors.lastname: Goldring
authors.forename: Christina
authors.initials: C
authors.lastname: Whittle
authors.forename: Shelley
authors.initials: S
authors.lastname: Patterson
authors.forename: Debopam
authors.initials: D
authors.lastname: Chakrabarti
authors.forename: Christian
authors.initials: C
authors.lastname: Doerig
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000378:metabolism; D016213:Cyclins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D016054:DNA, Protozoan / Q000235:genetics; D004912:Erythrocytes / Q000469:parasitology; D018507:Gene Expression Regulation, Developmental; D017125:Genes, Protozoan; D006801:Humans; D066298:In Vitro Techniques; D016778:Malaria, Falciparum / Q000469:parasitology; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010963:Plasmodium falciparum / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism; D015800:Protozoan Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
chemical_list: D018797:Cell Cycle Proteins; D016213:Cyclins; D016054:DNA, Protozoan; C406811:PfPK5 protein, Plasmodium falciparum; D015800:Protozoan Proteins; C486530:SPDYA protein, human; D011494:Protein Kinases; C102333:histone H1 kinase