||Many growth factors stimulate cell proliferation through mitogenic pathways such as the Ras/Raf/MEK/ERK cascade. Defective Ras signaling has long been associated with cancer progression and maintenance. Recently, numerous RGS proteins such as RGS1, RGS2, RGS4, RGS6, RGS17 and RGS19 have been shown to regulate cell proliferation, differentiation as well as cancer progression. The RGS proteins deactivate heterotrimeric G proteins rather than the Ras-like small GTPases. RGS19, also named as Gα-interacting protein (GAIP), is a RGS protein which acts as a GTPase activating protein (GAP) for Gαi/o and Gαq subunits. Being a RGS protein with multiple functional domains, RGS19 is capable of interacting with a variety of signaling molecules including those beyond G protein signaling. For instance, RGS19 binds to GIPN (GAIP-interacting N-terminal) and GIPC (GAIP-interacting C-terminal) to regulate protein degradation and growth factor signaling, respectively. Given that RGS19 stimulated cell proliferation in HEK 293 cells and exhibited a severely impaired Ras/Raf/MEK/ERK signaling, it would be extremely interesting to test the role of RGS19 in tumorigenesis, in terms of whether it has oncogenic potential or tumor suppressive effect. In this study, upon coexpression of RGS19, but not RGSZ1 (which belongs to the same subfamily as RGS19, and shares a high sequence homology) or RasGAP (a well-defined GAP protein for Ras), all of the hallmarkers of neoplastic transformation induced by oncogenic RasGV (G12V mutant), including an increased cell growth rate, foci formation, anchorage-independent growth, and tumorigenesis in nude mice were significantly suppressed. Additionally, the knock down of endogenous RGS19 significantly enhanced the tumorigenic ability of human lung cancer H1299 cells. Moreover, RGS19 significantly suppressed oncogenic Src (Y527F mutant)-induced tumorigenesis through attenuation of Src, Ras, Jak activity by forming a complex in a Src kinase activity-dependent manner. Furthermore, the Ras/Raf/MEK/ERK signaling, which plays a key role in oncogenic Ras-mediated carcinogenesis, was significantly impaired by RGS19. Taken together, these results suggest that RGS19 specifically inhibited oncogene-induced neoplastic transformation both in vitro and in vivo, as well as attenuated Ras/Raf/MEK/ERK and Src/Jak signaling. Other than the tumor-suppressive effect on oncogene-mediated transformation, RGS19 enhanced the growth rate of NIH3T3 cells, but not for RGSZ1. RGS19 appeared to participate in the proliferation of human cancer cells, since the knock down of endogenous RGS19 significantly reduced the growth rate of H1299 and HeLa cells. Interestingly, proteasome inhibitor MG132 and Lactacystin inhibited the degradation of both RGS19 and RGSZ1. The pull down of RGS19 and RGSZ1 resulted in the co-immunoprecipitation of poly-ubiquitin, while the presence of xp-GIPN did not affect the degradation of RGS19. Thus RGS19 was degraded through an ubiquitin-dependent proteasome pathway, but GIPN was not involved in the degradation of RGS19. The present study provided a highly interesting finding that RGS19, a negative regulator of G protein signaling, has inhibitory effect on oncogene-mediated neoplastic transformation. These results support the notion that RGS proteins are multi-functional and they do not simply serve as GAPs for Gα proteins.