| Modification Notes |
A critical step in signal transduction responses to stimulation of cell surface receptors by their ligands involves the accumulation of Ras proteins in their active GTP-bound state (1,2). To reach their active GTP-bound state, Ras proteins must first release bound GDP, a rate-limiting step mediated by a guanine nucleotide releasing factor (GRF) (3,4). The mammalian Ras p21 GRF protein has been designated Ras-GRF p140 (5). Ras-GRF accelerates release of GDP from H- and N-Ras p21 protein in vitro, but not from the related Ral A or Cdc42Hs GTP-binding proteins. Of interest, a region mapping within the amino terminal domain of Ras-GRF is similar to both the human breakpoint cluster protein, Bcr, and the Dbl proto-oncogene product, a guanine nucleotide-releasing factor for CDC42Hs (6,7). Ras-GRF2 p135 has also been identified. Ras-GRF2 p135 is highly homologous to Ras-GRF1 p140 except in the region between the REM and CDC25 domains (8) and appears to function similarly to Ras-GRF1 p140. |
