A novel 25 K Ras-like protein, Rin, binds calmodulin in a Ca2+-dependent manner and is considered to participate in a calcium/calmodulin-mediated signaling pathway. However, little is known about Rin signaling mechanism. Here we examined the signal transduction pathway through Rin protein using pull down assay system. When we stimulated Rin-expressed Cos-7 cells with ionomycin, phorbol 12-myristate 13-acetate or epidermal growth factor (EGF), Rin protein was rapidly activated. Moreover, cells cotransfected with Rin and mSos showed a similar Rin activation profile, and Rin protein was coimmunoprecipitated with mSos protein in vivo. When cells were cotransfected with Rin and Ras-GTPase-activating proteins, basal Rin activity was decreased. Association with Rin and calmodulin was potentiated after stimulation and Rin activation was regulated by both calcium ion and calmodulin. These studies suggest that Rin may be involved in EGF receptor and mSos-mediated signaling pathway and may participate in calcium/calmodulin-mediated cellular processes. (C) 2002 Elsevier Science (USA). All rights reserved.