All in the family? New insights and questions regarding interconnectivity of Ras, Rap1 and Ral

Ras, Rap1 and Ral are related small GTPases, While the function of Ras in signal transduction is well established, it has been recognized only recently that Rap1 and Ral also are activated rapidly in response to a large variety of extracellular signals, Between the three GTPase an intriguing interconnectivity exists, in that guanine nucleotide exchange factors for Ral associate with the GTP-bound form of both Ras and Rap1, Furthermore, Rap1 is considered to function as an antagonist of Ras signalling by trapping Ras effecters in an inactive complex. Here, I summarize the recent developments in understanding the functional relationship between these three GTPase and argue that Rap1 functions in a signalling pathway distinct from Ras, while using similar or identical effecters.