Previous studies have shown that G-proteins are multimeric structures with many structural and functional similarities to cytoskeletal proteins such as actin and tubulin. Also found in cells are punctate forms of G- proteins associated with stress fibers in at least two cell types; similar findings have been reported by others. Further complexity is revealed by findings that G-proteins and the effector signalling enzyme, adenylyl cyclase, are sequestered and taken into vesicular or endocytic structures that are internalized and which fuse with other vesicular elements. In CHO cells, the newly internalized structures do not return to the cell membrane but appear to be sequestered within the cell. However, when muscarinic receptors (transfected into the cells) are exposed to agonists, the endocytic process is dramatically and rapidly altered in that the endocytic vesicles return to the surface membrane with renewed localization of the G-proteins. The muscarinic signal-ling process interferes with early fusion steps that culminate in quasi- irreversible internal sequestration. Similar studies with rat adipocytes demonstrate a similar phenomenen when exposed to isoproterenol or ACTH. These findings suggest that signalling may be carried out not only at the surface membrane but also by the internalized endocytic vesicle. New approaches to the study of the hormonally controlled endocytic processes are needed. To this end, 3T3-L1 cell lines adapted to differention to adipocytes will be employed along with specific antibodies to the components of the signal transduction elements. Since these cells have the essential elements seen with isolated rat adipocytes it is planned to follow, using various techniques such as confocal microscopy, the fate of the internalized G-proteins and the dynamics of the endocytic process under different hormonal conditions, including the actions of lipolytic hormones and insulin.
| Putney Jr, J W (1999) ""Kissin' cousins"": intimate plasma membrane-ER interactions underlie capacitative calcium entry. Cell 99:5-8 |
