In vivo coupling of insulin-like growth factor II/mannose 6-phosphate receptor to heteromeric G proteins: Distinct roles of cytoplasmic domains and signal sequestration by the receptor

We examined the signaling function of the IGF-II/mannose 6-phosphate receptor (IGF-IIR) by transfecting IGF-IIR cDNAs into COS cells, where adenylyl cyclase (AC) was inhibited by transfection of constitutively activated Gα(i) cDNA (Gα(i2)Q205L). In cells transfected with [GF-IIR cDNA, IGF-II decreased cAMP accumulation promoted by cholera toxin or forskolin. This effect of IGF-II was not observed in untransfected cells or in cells transfected with IGF-IIRs lacking Arg²⁴¹⁰-Lys²⁴²³. Thus, IGF-IIR, through its cytoplasmic domain, mediates the G(i)-linked action of IGF-II in living cells. We also found that IGF-IIR truncated with C-terminal 28 residues after Ser²⁴²⁴ caused Gβγ-dominant response of AC in response to IGF-II by activating G(i). Comparison with the Gα(i)-dominant response of AC by intact IGF-IIR suggests that the C-terminal 28-residue region inactivates Gβγ. This study not only provides further evidence that IGF- IIR has IGF-II-dependent signaling function to interact with heteromeric G proteins with distinct roles by different cytoplasmic domains, it also suggests that IGF-IIR can separate and sequestrate the Gα and Gβγ signals following G(i) activation.