Modular construction of a signaling scaffold: MORG1 interacts with components of the ERK cascade and links ERK signaling to specific agonists.

Signal transduction occurs by the reversible assembly of oligomeric protein complexes that include both enzymatic proteins and proteins without known enzymatic activity. These nonenzymatic components can serve as scaffolds or anchors and regulate the efficiency, specificity, and localization of the signaling pathway. Here we report the identification of MORG1 (mitogen-activated protein kinase organizer 1), a member of the WD-40 protein family that was isolated as a binding partner of the extracellular signal-regulated kinase (ERK) pathway scaffold protein MP1. MORG1 specifically associates with several components of the ERK pathway, including MP1, Raf-1, MEK, and ERK, and stabilizes their assembly into an oligomeric complex. MORG1 facilitates ERK activation when cells are stimulated with lysophosphatidic acid, phorbol 12-myristate 13-acetate, or serum, but not in response to epidermal growth factor. Suppression of MORG1 by short interfering RNA leads to a marked reduction in ERK activity when cells are stimulated with serum. We propose that MORG1 is a component of a modular scaffold system that participates in the regulation of agonist-specific ERK signaling.