Unusual Interplay of Two Types of Ras Activators, RasGRP and SOS, Establishes Sensitive and Robust Ras Activation in Lymphocytes

Abstract

Ras activation is crucial for lymphocyte development and effector function. Both T and B lymphocytes contain two types of Ras activators: ubiquitously expressed SOS and specifically expressed Ras guanyl nucleotide-releasing protein (RasGRP). The need for two activators is enigmatic since both are activated following antigen receptor stimulation. In addition, RasGRP1 appears to be dominant over SOS in an unknown manner. The crystal structure of SOS provides a clue: an unusual allosteric Ras-GTP binding pocket. Here, we demonstrate that RasGRP orchestrates Ras signaling in two ways: (i) by activating Ras directly and (ii) by facilitating priming of SOS with RasGTP that binds the allosteric pocket. Priming enhances SOS' in vivo activity and creates a positive RasGTP-SOS feedback loop that functions as a rheostat for Ras activity. Without RasGRP1, initiation of this loop is impaired because SOS' catalyst is its own product (RasGTP)—hence the dominance of RasGRP1. Introduction of an active Ras-like molecule (RasV12C40) in T- and B-cell lines can substitute for RasGRP function and enhance SOS' activity via its allosteric pocket. The unusual RasGRP-SOS interplay results in sensitive and robust Ras activation that cannot be achieved with either activator alone. We hypothesize that this mechanism enables lymphocytes to maximally respond to physiologically low levels of stimulation.

We express our gratitude to Athena Lin and Martin McMahon for RasV12 and variants, Michael Karin for SOS1 and SOS1-F constructs, and Xiao-Ping Zhong and Gary Koretzky for the DGKζ construct, as well as Jim Stone and Stacey Stang for anti-RasGRP1 antibody. We are grateful to Susan Levin, Tomas Brdicka, Henry Bourne, and Martin McMahon for critically reading the manuscript. We thank Holger Sondermann, John Kuriyan, and the members of the Weiss lab for their suggestions and comments and S.L., M.M., T.E., and B.F. for Sunday cell culture.

J.R. is grateful for a grant from the Dutch Cancer Society (KWF). This work was supported in part by a grant from the NCI (CA 72531).