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[摘要]：Cell-surface TGF beta (transforming growth factor) receptors partition into membrane rafts and the caveolin-positive endocytic compartment by an unknown mechanism. In the present Study, we investigated the determinant in the TGF beta type II receptor (T beta RII) that is necessary for membrane raft/caveolar targeting. Using subcellular fractionation and immunofluorescence microscopy techniques, we demonstrated that the extracellular domain of T beta RII mediates receptor partitioning into raft and caveolin-positive membrane domains. Pharmacological perturbation of glycosylation using tunicamycin or the mutation of Mgat5 [mannosyl(alpha-1,6)-glycoprotein beta-1,6-N-acetylglucosaminyltransferase V] activity interfered with the raft partitioning of T beta RII. However, this was not due to the glycosylation state of T beta RII, as a non-glycosylated T beta RII mutant remained enriched in membrane rafts. This suggested that other cell-surface glycoproteins associate with the extracellular domain of T beta RII and direct their partitioning in membrane raft domains. To test this we analysed a GMCSF (granulocyte/macrophage colony-stimulating factor)-T beta RII chimaeric receptor, which contains a glycosylated GMCSF extracellular domain fused to the transmembrane and intracellular domains of T beta RII. This chimaeric receptor was found to be largely excluded from membrane rafts and caveolin-positive structures. Our results indicate that the extracellular domain of T beta RII mediates receptor partitioning into membrane rafts and efficient entrance into caveotin-positive endosomes. |
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