Interaction of Rac Exchange Factors Tiam1 and Ras-GRF1 with a Scaffold for the p38 Mitogen-Activated Protein Kinase Cascade

Abstract

Tiam1 and Ras-GRF1 are guanine nucleotide exchange factors (GEFs) that activate the Rac GTPase. The two GEFs have similar N-terminal regions containing pleckstrin homology domains followed by coiled-coils and additional sequences that function together to allow regulated GEF activity. Here we show that this N-terminal region of both proteins binds to the scaffold protein IB2/JIP2. IB2/JIP2 is a scaffold for the p38 mitogen-activated protein (MAP) kinase cascade because it binds to the Rac target MLK3, the MAP kinase kinase MKK3, and the p38 MAP kinase. Expression of IB2/JIP2 in cells potentiates the ability of Tiam1 or Ras-GRF1 to activate the p38 MAP kinase cascade but not the Jnk MAP kinase cascade. In addition, Tiam1 or Ras-GRF1 binding to IB2/JIP2 increases the association of the components of the p38 MAP kinase signaling cassette with IB2/JIP2 in cells and activates scaffold-associated p38. These findings imply that Tiam1 and Ras-GRF1 can contribute to Rac signaling specificity by their ability to form a complex with a scaffold that binds components of one of the many known Rac effector pathways.

We thank John Collard and Marc Symons for Tiam1 clones, Roger Davis for human JIP1 and JIP2 clones, Richard Cerione for a Dbl clone, Gary Johnson for MKK3β, MKK6β, and MKK7 clones, Kathy Gallo for the MLK3 clone, Tom Roberts for the p38 and Jnk clones, and Mitchell Goldfarb for anti-IB2 antibody. We also thank K. Eric Paulson for helpful comments on the manuscript.

This work was supported by a PHS grant to L.A.F. from the NCI.