Human CDK5–cyclin B1 structures uncover a conserved mitotic CDK activation mechanism

Cyclin-dependent kinase 5 (CDK5), long considered atypical and activated by non-cyclin cofactors in neurons, also functions in mitosis. To resolve its activation mechanism during mitosis, we determined the first high-resolution crystal structures of CDK5–cyclin B1 in apo and nucleotide-bound states. Contrary to AlphaFold predictions, we find CDK5 structures unexpectedly mirrors CDK1-cyclin B1 assembly and activation, establishing CDK5 as a bona fide mitotic kinase acting in parallel with CDK1.