Figure 3

The emerging molecular mechanism of Met4 regulation by SCF^Met30. The mechanism of Met4 activation by SCF^Met30 is based on the prediction that the formation of Met4 homodimers antagonizes proper assembly between Met4 and cofactors (A), and that the dissociation of Met4 homodimers by SCF^Met30 (B, step 1) is necessary to stabilize and/or rearrange the protein-protein interactions within the Met4/Met28/Cbf1 complex, triggering its proper assembly (B). The low abundance of SCF^Met30 suggests that proteolysis of the remaining Met4 molecule occupying SCF^Met30 (B, step 1) is necessary for SCF^Met30 recycling (B, step 2). As a result, 'two stepping' with SCF^Met30" could be necessary for each round of Met4 activity at a promoter (all steps involving SCF^Met30 are marked in blue). The stabilizing effect of cofactors on the SCF^Met30-Met4 interaction ('tight complex', note exposure of the UIM domain in Met4) allows Met4 inhibition by polyubiquitination only (C), unless methionine (D) or cysteine (D^REP) is available to destabilize the tight interaction between Met4 and SCF^Met30, allowing Met4 proteolysis. Disassembly of the Met4 complex by the proteasome could link activation of methionine biosynthesis to cell division by releasing the DNA binding cofactors Cbf1 and Met31/32 from promoters (D), which, at least in the case of Cbf1, would make it available for its cell division role. Dissociation of Met4 prior to its proteolysis could protect the cofactors and SCF^Met30 from effects of the proteasome-mediated disassembly (D^REP), preventing Cbf1 and/or Met31/32 release. See text for details.