The low density lipoprotein receptor-related proteindeleted in tumor (LRP1B, initially referred to as LRP-DIT) was cloned and characterized as a candidate tumor suppressor. It is a new member of the low density lipoprotein receptor gene family. Its overall domain structure and large size (ϳ600 kDa) are similar to LRP and suggest that it is a multifunctional cell surface receptor. Herein, we characterize a series of ligands for the receptor using cell lines that stably express it as a domain IV minireceptor (mLRP1B4). Ligands of LRP including receptor-associated protein, urokinase plasminogen activator, tissue-type plasminogen activator, and plasminogen activator inhibitor type-1 each demonstrate binding, internalization, and degradation via mLRP1B4. Interestingly, the kinetics of ligand endocytosis is distinctly different from that of LRP, with LRP1B exhibiting a markedly diminished internalization rate. In addition, tissue expression analysis reveals that the LRP1B gene is expressed in brain, thyroid, and salivary gland. These studies thus extend the physiological roles of members of the LDL receptor family.Members of the low density lipoprotein receptor gene family play a wide variety of roles in normal cell function and development. For example, mutations of the low density lipoprotein receptor gene, the prototypic family member, result in the genetic disease familial hypercholesterolemia (1). Much attention has focused on other members of this gene family (including LRP, 1 megalin, apolipoprotein E receptor-2, very low density lipoprotein receptor) because of their recently recognized roles in development, cell signaling, and pathogenesis (2, 3). The low density lipoprotein receptor-related protein LRP1B (initially referred to as LRP-DIT (deleted in tumor)), a candidate tumor suppressor, is a new member of the giant receptor subgroup of this gene family. It is located at chromosome 2q21.2 and was isolated by positional cloning based on homozygous deletions detected in human cancer cell lines (4 -6). Mutation analysis revealed that the gene is frequently (45%) inactivated in human non-small cell lung cancer cell lines by intragenic homozygous deletions, point mutations, and aberrant transcripts missing internal portions. Loss of heterozygosity analysis also detected high allelic loss within the locus using two independent microsatellite polymorphism markers, suggesting that the gene is a candidate tumor suppressor (4). The low density lipoprotein receptor gene family previously contained two very large members, LRP (LRP1), a dimer of 515 and 85 kDa, and its closely related homolog, megalin (LRP2), a single species of ϳ600 kDa (2, 3). LRP1B is more closely related to LRP (with similarity of 59 and 52% identity at the cDNA and predicted amino acid levels, respectively) than to LRP2 (4). The cDNA of LRP1B is 16.5 kilobases, with an open reading frame of 13,797 base pairs, which encodes a protein of 4599 amino acids. Characterization of exon-intron boundaries determined that the genomic DNA of LRP1B contains 91 e...

The LDL receptor-related protein (LRP) is a large, multifunctional endocytic receptor that binds and endocytoses a variety of structurally and functionally distinct ligands. LRP contains four putative ligand-binding domains. However, only domains II, III and IV, but not domain I, bind the receptor-associated protein (RAP), a molecular chaperone and universal antagonist for LRP. In order to dissect the function of RAP in LRP folding and to examine the ligand-binding properties of LRP, we generated LRP minireceptors that represent each of the four putative ligand-binding domains (termed mLRP1, mLRP2, mLRP3 and mLRP4, respectively). We found that proper folding and trafficking of mLRP2, mLRP3, mLRP4, but not mLRP1, is facilitated by coexpression of RAP. When these mLRPs were stably expressed in Chinese Hamster Ovary cells that lack the endogenous LRP, we found that each of these receptors was processed and traffics through the secretory pathway. Cell surface expression of these minireceptors was quantitatively examined by flow cytometric analyses. Using these minireceptor cell lines to map the ligand-binding domains, we found that although the majority of LRP ligands bind to both domain II and domain IV, Pseudomonas exotoxin A utilizes only domain IV for its binding to LRP. We conclude that while domains II and IV of LRP share many ligand-binding properties, each of the putative ligand-binding domains of LRP is unique in its contribution to ligand binding.