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 G alpha(i), cDNA (G alpha(i2)Q205L). In cells transfected with IGF-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 Arg(2410)-Lys(2423). 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 Ser(2424) caused G beta gamma-dominant response of AC in response to IGF-II by activating G(i). Comparison with the G alpha(i)-dominant response of AC by intact IGF-IIR suggests that the C-terminal 28-residue region inactivates G beta gamma. 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 G alpha and G beta gamma signals following G(i) activation.