In Saccharomyces cerevisiae, the pheromone-induced ubiquitylation and degradation of the filamentation pathway-specific activator, Tec1, suppresses cross talk between the mating and filamentous growth mitogen-activated protein kinase (MAPK) pathways. The mating pathway MAPK, Fus3, phosphorylates Tec1, resulting in its recognition by the SCF (for Skp1, Cullin, F-box containing) E3 ubiquitin ligase complex, leading to its proteolysis. Previously, it was found that Tec1 destruction requires phosphorylation on threonine 273 (T273). T273 is embedded in the sequence LLpTP, which is identical to the canonical binding site for Cdc4, a conserved F-box substrate adaptor for the SCF complex. However, recent work on both Cdc4 and the human Cdc4 ortholog Fbw7 has shown that a second substrate phosphorylation can be required for optimal Cdc4 binding in vitro. We report here that high-affinity binding of recombinant Cdc4 to Tec1 phosphopeptides requires phosphorylation of not only T273 but also a second site, T276. Significantly, both phospho-sites on Tec1 and a conserved basic pocket on Cdc4 are critical for Tec1 proteolysis in response to pheromone treatment of cells, establishing a role for two-phosphate recognition by yeast Cdc4 in substrate targeting in vivo.