A Novel Cytosolic GTP-Binding Protein with Phospholipid Stimulated GTP-Binding And GTPase Activity
The signal transducing GTP-binding proteins (G-proteins) isolated thus far are membrane associated. However, rapidly accumulating data suggest that regulatory G-proteins may also be cytosolic. This conclusion is based mainly on indirect observations such as the ability of cytosol to restore adenylate cyclase activity of cyc- membranes (1), the presence of cholera or pertussis toxins substrates in the cytosol (2,3) and the activation of a soluble phosphoirositide-hydrolyzing PLC by GTP-ɣ-S (4). The firadings that the purified α-subunits of Go and Gi are water soluble (5), that oc-subunits of Gs are released from the membrane following activation (6) and that about 80% of the GTP-binding protein ARF is located in the cytosolic fraction (7) further support his notion. Indeed, it has been recently argued that the critical interactions of the signal transducing G-proteins occur in the cytoplasm rather than in the membrane (8). In addition, several reports (9–11) have implicated GTP-binding proteins as regulatory elements in the transport and secretion of proteins. These data again imply that the distribution and function of G-proteins should be extended from the surface of the cell to the interior (12).
KeywordsCytosolic Fraction Guanine Nucleotide Diphteria Toxin Pertussis Toxin Substrate Binding Polypeptide Chain
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