BINDING AND ENDOCYTOSIS OF 39 KDA PROTEIN BY MDBK CELLS

A 39 kDa protein copurifies with the low density lipoprotein receptor- related protein (LRP) and regulates ligand interactions with LRP. In o ur recent studies on the clearance of the 39 kDa protein in vivo, we d emonstrated that once the liver LRP receptors were saturated, the kidn ey 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 cell s. Herein we demonstrate specific, high-affinity, saturable, and Ca2+- dependent binding of the I-125-39 kDa protein to MDBK cells (K-d simil ar to 10-15 nM, 50-70,000 binding sites per cell). Cellular uptake and degradation of the I-125-39 kDa protein by MDBK cells was also demons trated with kinetics typical of receptor-mediated endocytosis. Using c hemical crosslinking we show that LRP in part mediates the binding of I-125-39 kDa protein to the MDBK cell surface. In addition, the presen ce of functional LRP on the MDBK cell surface was confirmed by the spe cific binding of activated alpha(2)-macroglobulin, another ligand of L RP. Our data thus demonstrate the ability of kidney-derived MDBK cells to specifically bind, endocytose, and degrade the 39 kDa protein. (C) 1995 Wiley-Liss, Inc.