Functional regulation of the tubby family proteins. (a) The IFT-A core recruits TULP3 by binding to its IFT-A binding region in the amino terminus. The TULP3 amino-terminal fragment inhibits TULP3 loading to the IFT-A complex. The IFT-A-TULP3 interaction gates ciliary GPCRs by PIP2 binding through TULP3's tubby domain. A full-length TULP3 with defective PIP2 binding is thought to inhibit loading of TULP3-IFT-A to the PIP2 vesicles . Thus, TULP3 bridges the membrane phosphoinositides and IFT-A complex to gate ciliary GPCR trafficking. (b) The tubby domain is attached to membrane PIP2. Following activation of Gαq to Gαq* by GPCR activation, PIP2 hydrolysis causes the tubby domain to be dislodged from the membrane . IP3, inositol triphosphate; DAG, diacylglycerol; PLC, phospholipase C.