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 made by others with different cell lines. Further complexity is revealed by findings that G-proteins, protein kinases and phosphatases, and adenylyl cyclase, are derived from specialized regions of the plasma membrane that are encapsulated with caveolin, a protein that characterizes a family of structures called caveolae. In CHO cells transfected with muscarinic receptors, these receptors are taken into caveolae. Caveolae in these cells are pinched off and rapidly form endosomes that, through a complex process of fusion, eventually incorporate the contents into what are likely lysosomes for degradation. When the muscarinic receptors are activated, calcium release from stored sources rapidly increases and early stage fusion of primary caveolae is aborted, thus preventing processing and destruction of the signalling devices. A major result is that the endosomes carrying the signalling devices (G-proteins, adenylyl cyclase, phospholipases, etc.) are cycled back to the plasma membrane. Current research efforts are designed to investigate the dynamics of the cycling process, the fate of the various signalling elements, and how the trafficking of the early endosomes formed from caveolae occurs, particularly with regard to the evidence that actin filaments seem to be essential for the trafficking and cycling of the hormone-sensitive processes necessary for signal transduction in cells.
| Putney Jr, J W (1999) ""Kissin' cousins"": intimate plasma membrane-ER interactions underlie capacitative calcium entry. Cell 99:5-8 |
