A 39 kDa protein copurifies with the low density lipoprotein receptor-related protein (LRP) and regulates ligand interactions with LRP. In our recent studies on the clearance of the 39 kDa protein in vivo, we demonstrated that once the liver LRP receptors were saturated, the kidney became the major organ responsible for the 39 kDa protein clearance (Warshawsky et al., 1993, J. Clin. Invest., 92:937-944). The current study was undertaken in order to investigate the potential binding and cellular processing of the 39 kDa protein by kidney-derived MDBK cells. Herein we demonstrate specific, high-affinity, saturable, and Ca*+-dependent binding of the '"1-39 kDa protein to MDBK cells (Kd -10-15 nM, 50-70,000 binding sites per cell). Cellular uptake and degradation of the 'L51-39 kDa protein by MDBK cells was also demonstrated with kinetics typical of receptormediated endocytosis. Using chemical crosslinking we show that LRP in part mediates the binding of '251-39 kDa protein to the MDBK cell surface. In addition, the presence of functional LRP on the MDBK cell surface was confirmed by the specific binding of activated a,-macroglobulin, another ligand of LRP. Our data thus demonstrate the ability of kidney-derived MDBK cells to specifically bind, endocytose, and degrade the 39 kDa protein. o 1995 WiIey-Liss, Inc.