Kir 2.3 C-terminus specifically interacts with the novel PDZ protein TIP-1. (A) Yeast were cotransformed with AD-TIP-1 and LexA DNA-binding domain fusions of wild-type (WT) Kir 2.3 C-terminus (top), a mutant Kir 2.3 C-terminus, lacking the last 3 amino acids (ΔPDZ; middle), or bicoid, an unrelated Drosophila protein (bottom). Transfected yeast were diluted and spotted onto selection/reporter plates, X-gal or -Leu. Transcriptional activation of the lacZ reporter produces blue yeast on X-gal plates; Activation of leu2 reporter allows yeast to grow on “leucine drop-out” plates (-Leu). (B) Schematic of the TIP-1 protein compared with mLin-7. Atypical of PDZ proteins, TIP-1 contains a single protein–protein interaction domain.