Location of agonist‐dependent‐phosphorylation sites in the third intracellular loop of muscarinic acetylcholine receptors (m2 subtype)

Publisher Website
Google Scholar
Abstract
Muscarinic acetylcholine receptors (mAChR, human m2 subtype) expressed in Sf9 (Spodoptera frugiperda) cells using the baculovirus system were purified and subjected to phosphorylation by a mAChR kinase, which was partially purified from porcine cerebrum. Two bands with apparent molecular masses of 59 kDa and 39 kDa as determined by SDS/PAGE were found to be phosphorylated in an agonist-dependent manner. Both bands were labeled by the irreversible muscarinic ligand [3H]propylbenzilylcholine mustard. Molecular masses of the [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled bands decreased following treatment with N-glycanase. The 59-kDa and 39-kDa bands were converted to 52-kDa and 32-kDa bands, respectively, indicating that both the 59-kDa and 39-kDa bands contain the amino-terminal region where glycosylation sites are present. The ratio of incorporated [32P]phosphate and bound [3H]propylbenzilylcholine mustard was essentially the same for the 59-kDa and 39-kDa bands, indicating that all the phosphorylation sites reside in the sequence of 39 kDa from the amino-terminal region. The amounts of incorporated [32P]phosphate were estimated to be 10-11/receptor, with 7-8 serine and 3-4 threonine, but no phosphorylated tyrosine residues. Further treatment of [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled receptors with V8 protease indicated that the phosphorylation sites were not present in 30-kDa amino-terminal segment. These results indicate that the phosphorylation sites are localized in the range 30-39 kDa from the amino terminus, which consists of primarily the central part of the third intracellular loop. Consistent with this conclusion, a fusion protein containing glutathione S-transferase linked to a peptide corresponding to residues 227-324 of the central part of the third intracellular loop was found to be phosphorylated by the mAChR kinase in a heparin-sensitive manner.