Structural and functional relationship between the Ph1 locus protein 5B2 in wheat and CDK2 in mammals.

The Ph1 locus in hexaploid wheat is responsible for restricting chromosome pairing at meiosis to true homologues by suppressing homoeologous pairing. Based on detailed modelling studies and predicted ability to form complexes with cyclin-A and cyclin-dependent kinase inhibitor such as p27, Triticum aestivum-5B2 (( Ta ) 5B2) is suggested to be a wheat analogue of human CDK2 enzyme. A blast analysis of the protein data bank using the amino acid sequence of the protein expressed by the 5B2 copy of the cdk-like cluster of genes at the Ph1 locus (( Ta ) 5B2) identified humans CDK2 as a top hit. In this analysis, the canonical cyclin binding motif PSTAIRE of CDK2 is replaced by a novel DARTLRE motif and Thr160 residue, phosphorylation of which is required for positive regulation of CDK2, is replaced by a tyrosine (Tyr174) in ( Ta ) 5B2. Despite these differences, detailed analyses show that all residues known to be important for cyclin binding are either fully conserved or whenever there is alteration in ( Ta ) 5B2, a corresponding but comparable alteration is also observed in plant cyclins notably cyclin-A of Arabidopsis thaliana. Moreover, the Thr160/Tyr174 substitution is also accommodated by suitable alterations in the 3D space around Tyr174 and the 3D model of ( Ta ) 5B2 predicts Tyr174 to play the same role as Thr160 plays in CDK2.