Small GTPases of the Ras superfamily are master regulators of intracellular trafficking and constitute essential signaling components in all eukaryotes. Aberrant small GTPase signaling is associated with a wide spectrum of human diseases, including cancer. Here, we developed a high-throughput, multiple reaction monitoring-based workflow, coupled with stable isotope labeling by amino acids in cell culture, for targeted quantification of approximately 100 small GTPases in cultured human cells. Using this method, we investigated the differential expression of small GTPases in three pairs of primary and metastatic melanoma cell lines. Bioinformatic analyses of The Cancer Genome Atlas data and other publicly available data as well as cell-based assays revealed previously unrecognized roles of RAB38 in promoting melanoma metastasis. Diminished promoter methylation and the subsequent augmented binding of transcription factor MITF contributed to elevated expression of gene in metastatic versus primary melanoma cells. Moreover, RAB38 promoted invasion of cultured melanoma cells by modulating the expression and activities of matrix metalloproteinases-2 and -9. Together, these data establish a novel targeted proteomic method for interrogating the small GTPase proteome in human cells and identify epigenetic reactivation of RAB38 as a contributing factor to metastatic transformation in melanoma. A novel quantitative proteomic method leads to the discovery of RAB38 as a new driver of metastasis in melanoma. .
