2 DISTANTLY POSITIONED PDZ DOMAINS MEDIATE MULTIVALENT INAD-PHOSPHOLIPASE-C INTERACTIONS ESSENTIAL FOR G-PROTEIN-COUPLED SIGNALING

Drosophila INAD, which contains five tandem protein interaction PDZ do mains, plays an important role in the G protein-coupled visual signal transduction, Mutations in InaD alleles display mislocalization of sig naling molecules of phototransduction which include the essential effe ctor, phospholipase C-beta (PLC-beta), which is also known as NORPA, T he molecular and biochemical details of this functional link are unkno wn. We report that INAD directly binds to NORPA via two terminally pos itioned PDZ1 and PDZ5 domains. PDZ1 binds to the C-terminus of NORPA, while PDZ binds to an internal region overlapping with the G box-homol ogy region (a putative G protein-interacting site). The NORPA proteins lacking binding sites, which display normal basal PLC activity, can n o longer associate with INAD in vivo. These truncations cause signific ant reduction of NORPA protein expression in rhabdomeres and severe de fects in phototransduction. Thus, the two terminal PDZ domains of INAD , through intermolecular and/or intramolecular interactions, are broug ht into proximity in vivo. Such domain organization allows for the mul tivalent INAD-NORPA interactions which are essential for G protein-cou pled phototransduction.