THE LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN (LRP) IS PROCESSED BY FURIN IN-VIVO AND IN-VITRO

The low-density-lipoprotein receptor-related protein (LRP) is a multif unctional receptor involved in the clearance of a large number of dive rse ligands, including proteases, protease-inhibitor complexes and lip oproteins. The mature receptor is composed of a 515 kDa and a 85 kDa s ubunit generated by proteolytic cleavage from a 600 kDa precursor poly peptide in a trans-Golgi compartment. Proteolytic processing occurs C- terminal to the tetrabasic amino acid sequence RHRR, a consensus recog nition site for precursor processing endoproteases or convertases. In this study we have identified furin, a subtilisin-type protease, to be necessary for efficient processing of LRP in cells. Furin-deficient R PE.40 cells exhibited an impaired processing of endogenous LRP and of a recombinant soluble form of the receptor containing the processing s ite. The processing defect in RPE.40 cells could be complemented by ex pression of furin from a transfected cDNA in cultured cells and by pur ified furin in vitro. The impaired maturation of LRP in RPE.40 cells d id not affect its intracellular transport, and correlated with a sligh t but consistent reduction in the endocytosis of LRP-specific ligands. These data suggest that proteolytic processing of LRP by furin is not necessary for intracellular trafficking but might be required for nor mal receptor activity.