IN-VIVO COUPLING OF INSULIN-LIKE GROWTH-FACTOR-II MANNOSE 6-PHOSPHATERECEPTOR 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 r eceptor (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 wi th IGF-IIR cDNA, IGF-II decreased cAMP accumulation promoted by choler a toxin or forskolin. This effect of IGF-II was not observed in untran sfected 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 bet a 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 I GF-IIR suggests that the C-terminal 28-residue region inactivates G be ta gamma. This study not only provides further evidence that IGF-IIR h as 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.