Laminin-5 (alpha3 beta3 gamma2) is an important component of epithelial basement membranes. The 190-kDa alpha3 chain undergoes extracellular cleavage within the carboxyl (C) terminus consisting of five globular domains (G1 to G5), producing the mature laminin-5 with the 160-kDa alpha3 chain. To understand the physiological meaning of this processing, we isolated the C-terminal fragments of the alpha3 chain from the conditioned media of two kinds of human cell lines. The amino-terminal sequence of the fragments suggested that the cleavage occurs at Gln(1337)-Asp(1338) in the spacer region between the G3 and G4 domains. The G4-G5 fragment itself did not show significant activity, but it stimulated cell migration in the presence of a low concentration of the mature laminin-5, suggesting its regulatory role in cell migration. (C) 2000 Academic Press.