Partial characterization of mitochondrial G proteins in adrenal cells

AbstractFour low molecular mass G proteins have been identified in mitochondrial membranes from bovine adrenal cortex. These proteins (referred to as proteins 1 to 4) showed molecular masses of 28, 27, 26 and 24 kDa with isoelectric points (pI) of 8.1, 5.6, and 6.3 respectively for proteins 1, 2 and 4. Protein 3 was shown to be heterogeneous, with isoelectric points of 5.0–6.1. Proteins were identified by binding of [α-32P]guanosine triphosphate (GTP) after separation by 12% SDS-polyacrylamide gel electrophoresis and transfer to nitrocellulose. Competitive binding by unlabelled competing nucleoside phosphate ligands showed specificity for guanosine triphosphate (GTP) and guanosine diphosphate (GDP) with little binding of guanosine monophosphate and no detectable binding with adenosine nucleoside phosphates. Binding was less than 10% with 100-fold excess GDP and GTP which showed equal intensities of binding. Inhibition of binding by 1000-fold cytidine triphosphate and uridine triphosphate was approx. 10%. Magnesium (Mg2+) stimulated binding of GTP by all four proteins. The effect of Mg2+ was essentially the same for proteins 1, 2 and 3, while protein 4 was less sensitive to Mg2+ at concentrations