CDKd1

CDKd1, short for CDK-activating kinase subunit 1, is a cyclin-dependent kinase-activating kinase found in plants, most notably in Arabidopsis thaliana. It belongs to the CDKD family of CDK-activating kinases and is homologous to CAKs in other eukaryotes. The protein functions as part of a CAK complex that activates cyclin-dependent kinases by phosphorylation of their T-loop, enabling cell cycle progression and, in many organisms, transcriptional control.

Biochemical function: CDKD1 phosphorylates the activation loop of CDKs on a threonine residue, licensing them for catalytic activity. In plants, CAK activity is associated with a Cyclin H-like partner (often referred to as CYCH) and a MAT1-like regulatory subunit; together these components form the plant CAK that integrates cell cycle regulation with transcriptional control through phosphorylation of the RNA polymerase II CTD.

Genetic and expression data indicate that CDKD1 is part of a gene family with related enzymes in plants, including CDKD2 and CDKD3. Expression tends to be broad, with activity in meristems and developing tissues, consistent with roles in growth and development. Loss- or reduction-of-function mutants show effects on growth and cell division, illustrating the importance of CAK activity in plant development, though functional redundancy with other CDKD family members is common.

Evolutionarily, CDKD proteins are conserved across eukaryotes as CAKs, aligning with the broadly conserved mechanism of CDK activation by phosphorylation. In plants, CDKD1 is one member of a CAK module that mirrors the CDK7/Cyclin H/MAT1 complex seen in animals, highlighting the conserved nature of CDK regulation while allowing plant-specific adaptations.