is overexpressed in multiple tumors, leading to the widely held view that this gene drives tumor progression, but this hypothesis has not been rigorously tested in melanoma. Here, we combined a conditional knockout of Coronin 1C with a genetically engineered mouse model of PTEN/ BRAF-driven melanoma. Loss of Coronin 1C in this model increases both primary tumor growth rates and distant metastases. Coronin 1C-null cells isolated from this model are more invasive in vitro and produce more metastatic lesions in orthotopic transplants than Coronin 1C-reexpressing cells due to the shedding of extracellular vesicles (EVs) containing MT1-MMP. Interestingly, these vesicles contain melanosome markers suggesting a melanoma-specific mechanism of EV release, regulated by Coronin 1C, that contributes to the high rates of metastasis in melanoma. Melanoma is the deadliest form of skin cancer with a high propensity for metastatic spread 1,2. The most common genetic drivers of melanoma are BRAF-activating mutations such as V600E, often found in conjunction with loss of tumor suppressors such as PTEN 3-5. If caught in its early stages, melanoma is treatable by surgical resection, but prognosis worsens significantly with the occurrence of in-transit, regional, or distant metastasis. Clinically, up to 54% of metastatic melanoma patients show tumor dissemination to the brain, 77% to the liver, and 85% to the lung 6. The high rate of metastasis in melanoma is poorly understood, but may be linked to lineage-specific factors that impact vesicular trafficking, secretion, degradation, and overall cell migration 7,8. The extracellular matrix (ECM) is an important substrate for tumor cell migration, but it also acts as a physical barrier whose degradation by matrix metalloproteinases (MMPs) is thought to be a critical step in tumor dissemination 9-11. MMPs are a family of both transmembrane (designated "membrane-type" or "MT") and secreted catalytic enzymes capable of cleaving ECM proteins and other substrates. Membrane-type 1 matrix metalloproteinase (MT1-MMP) is a particularly important pro-invasive MMP across many cancer types 12 and whose expression closely correlates with invasion and metastasis 13-15. MT1-MMP is also unique in its ability to both directly cleave a wide range of ECM proteins including collagen types I, II, and III, laminin 1 and 5, and fibronectin, as well as activate pro-MMP2 16. The trafficking of MT1-MMP to and from the plasma membrane (PM) and other membrane structures is surprisingly complex, but critical for the protein's function during tumor invasion and metastasis. Phosphorylation of the cytoplasmic tail initiates internalization of MT1-MMP from the PM by both clathrin-mediated and caveolin-mediated routes 13. Internalized vesicles traffic through the endolysosomal pathway, resulting in redirection to other areas of the PM or to the lysosome for degradation 17. MT1-MMP recycling has been shown to involve various flotilins 18 and SNARE proteins 19-22 , as well as a variety of Rab proteins including Rab5, Rab7 1...