Secreted lysosomal cysteine proteases (cathepsins) are involved in degradation and remodeling of the extracellular matrix (ECM) thus contributing to cell adhesion and migration. Among the eleven human lysosomal cysteine proteases only (pro)cathepsin X contains an RGD motif located in a highly exposed region of the propeptide which may allow binding of the proenzyme to RGD-recognising integrins. Here, we have tested procathepsin X for cell adhesive properties and found that it supports integrin avß3 - dependent attachment and spreading of endothelial cells (HUVEC). Using site-directed mutants of procathepsin X we proved that this effect is mediated by the RGD sequence within the proregion of the protease. Endogenous (pro)cathepsin X is transported to the plasma membrane, accumulates in vesicles at lamellipodia of HUVEC and is partly associated with the cell surface as shown by immunofluorescence. In addition, (pro)cathepsin X is partly colocalized with integrin ß3 as detected by immunogold electron microscopy. A direct interaction between endogenous procathepsin X and avß3 was demonstrated by co-immunoprecipitation. Moreover, surface plasmon resonance (SPR) analysis revealed significant and RGD-dependent binding of procathepsin X to integrin avß3. Our results provide for the first time evidence that the extracellular function of cathepsin X may include binding to integrins thereby modulating the attachment of migrating cells to ECM components.