Figure 3: Schematic of possible effects of zip1-phosphomimetic mutant (e.g., zip1-S75E [8]) on the Zip1-mediated coupling between homologous CENs and the centromeric SC formation. (a) Coupling between the homologous CENs is disrupted which, in turn, inhibits SC initiation from the CENs. Pericentromeric SC will be eventually formed by polymerization of Zip1 from the proximal crossover site (thick red arrow). (b) Coupling between the homologous CENs, but not centromeric SC initiation, is disrupted. Zip1 polymerization initiates from the CEN bound Zip1. Note that the homologous CENs appear to pair due to the secondary consequence of centromeric SC formation. (c) Zip1-dependent coupling between the homologous CENs, which is promoted also by Zip3, is unaffected. Additionally, Zip3 exerts two opposing effects on centromeric SC initiation (both (b) and (c)). Before DSB formation and crossover (CO) recombination, Zip3 inhibits centromeric SC initiation [25]. After DSB formation and initiation of meiotic recombination, Zip3 promotes centromeric SC initiation. A pair of homologous chromosomes, for simplicity each with one chromatid is shown. SC formation in one arm is shown. Lateral elements of SC and Rec8 are not shown. The direction of SC formation is shown by red arrows. A single CO per homologous pair is shown.