Growth Rate Dependence of Differential Incorporation of a Guanosine Triphosphate Photoaffinity Probe into the α Subunit of a Guanine Nucleotide Binding Protein, Gs, from Metastatic Variants of B16 Melanoma Cells

The signal transducing regulatory protein (G1α) was examined in B16 melanoma clones of low (F1C29) and high (F10C23) experimental metastatic potential. Incorporation of the photoaffinity analogue, [8-azido-γ-32P]GTP, into G1α was decreased in F10C23 extracts when compared to F1C29. This difference disappeared when the photolabeling reaction was carried out at an elevated temperature which enhanced the rate of GTP exchange, suggesting functional differences in the ability of Gsα to bind or release GTP rather than dissimilar intracellular Gsα concentrations. Differential Gsα photolabeling occurred only during the period of rapid growth when F10C23 cells proliferated faster than F1C29 cells. During the recovery phase of growth immediately following plating and at confluence, periods in which F1C29 and F10C23 growth rates are similar, Gsα photolabeling between the two clones was equal. CMT lung carcinoma clones of differential metastatic potential grew at a uniform rate at all stages of growth and also exhibited equal Gsα photolabeling. F10C23 cells were more responsive to α-melanocyte-stimulating hormone stimulation of adenylate cyclase activity than F1C29 cells at all growth stages. These results confirm previously observed functional differences in Gsα between B16 metastatic variants and show that photolabeling differences in Gsα are related to growth rate.