Updated project metadata. ADAMTS9 and ADAMTS20 are homologous secreted proteases implicated in ECM proteolysis and ciliogenesis, but few relevant substrates of these proteases are currently known. Quantitative N-terminomics comparison of RPE-1 cells lacking ADAMTS9 with parental RPE-1 cells identified transmembrane protease MT1-MMP (MMP14) as a novel ADAMTS9 substrate. The resulting enhanced cell-surface MT1-MMP activity in the gene-edited cells contributes to their adhesion defect, but not lack of cilia. A key physiological function of ADAMTS9/20 may be to dampen cell-surface MT1-MMP activity.