Figure 4.

The reaction network (chemical circuit) controlling MPF activity in frog egg extracts. MPF is a heterodimeric protein, consisting of a protein kinase subunit (Cdc2) and a regulatory subunit (CycB). In intact, fertilized frog eggs and in frog egg extracts, CycB is synthesized from amino acids at a constant rate (topmost ↓ arrow) and degraded in response to the periodic activation of the APC. CycB subunits combine with Cdc2 subunits (available in excess) to form active MPF (Cdc2 : CycB), but the active dimer is rapidly phosphorylated by Wee1 (a protein kinase). The cell (or extract) enters mitosis when inactive P-Cdc2 : CycB is dephosphorylated by Cdc25 (a protein phosphatase). Note that MPF activates Cdc25 and APC and inhibits Wee1. (Online version in colour.)