Figure 6.

Mad1p and Mad2p interact in the absence of other yeast proteins. Mad1p and Mad2p co-immunoprecipitate from COS cells co-transfected with MAD2 and MAD1. MAD2 was transfected into COS cells alone (lanes 1 and 4) or co-transfected with MAD1 (lanes 3 and 6) or with MAD1 and MPS1 (lanes 2 and 5) as indicated. Mad2p was immunoprecipitated from cell lysates and immunoblotted with an anti-Mad1p (lanes 1–3) or an anti-Mad2p (lanes 4–6) antibody. Both the unphosphorylated Mad1p and the Mps1-induced phosphorylated form co-immunoprecipitated with Mad2p. The migration of molecular size standards is indicated.