Fig. 8. A model for Perd function in formation of muscle attachments. (A) Muscle-expressed Perd protein forms a complex with Grip, directing it to sites of tendon contact. We hypothesize that PS1 integrin heterodimers (the product of the mew and mys genes) expressed on tendon cells may serve as the ligand for Perd binding and thus mediate target recognition. See Discussion for details. The additional PDZ domains of Grip can then recruit other proteins required for the maturation of the myotendinous junction; the PS2 integrin expressed on the muscle and known to be required for stable muscle attachment is an attractive candidate, but no direct interaction with Grip has been demonstrated. (B-D) Elements of this model have previously been reported (references, bottom right and in Discussion) in other systems where Perd orthologues are expressed, including interaction with an integrin of the laminin-binding class (B), interaction with Grip (C), and acting as an adhesion coreceptor for an integrin in cis (D).