Human cyclin A is adenovirus E1A-associated protein p60 and behaves differently from cyclin B.

CYCLINS are proteins synthesized during each cell cycle and abruptly destroyed in each mitosis. Cyclins have been implicated in the induction of mitosis and are associated with the serine-threonine protein kinase p34cdc2 as components of mitosis promoting factor (MPF). On the basis of conserved sequence motifs cyclins can be divided into A or B types. We recently cloned a human cyclin B and showed that cyclin B expression is regulated transcriptionally and post-translationally during the cell cycle, and that cyclin B associates with p34cdc2. Here we report that human cyclin A messenger RNA and protein levels also vary during the cell cycle, and increase and decrease in advance of cyclin B levels. Cyclin A is associated with a protein of relative molecular mass 33,000 that is related to, but distinct from, p34cdc2, and this complex has histone H1 kinase activity in vitro. Cyclin A is identical to p60, a protein that associates with p34cdc2 in interphase cells and with adenovirus E1A in transformed cells.