The somatic cell-specific low density lipoprotein receptor-related protein of the chicken. Close kinship to mammalian low density lipoprotein receptor gene family members.

Recently, the family of mammalian genes homologous to that for the low density lipoprotein (LDL) receptor has grown. One of the new family members, termed LDL receptor-related protein/alpha 2-macroglobulin receptor (LRP/alpha 2MR), is one of the largest cell surface proteins characterized to date. Its functions have been hypothesized to include the plasma clearance of chylomicron remnants and activated alpha 2-macroglobulin, as well as the local metabolism of complexes between plasminogen activators and their endogenous inhibitors. Here we describe the molecular characterization of an LRP/alpha 2MR expressed in chickens, which do not metabolize chylomicron remnants. This chicken protein is expressed in somatic tissues and is different from a second LRP/alpha 2MR exclusively expressed in growing ovarian follicles. The sequence of the somatic cell-specific chicken LRP/alpha 2MR, deduced from cloned full-length cDNA, shows 83% overall identity with human LRP/alpha 2MR. Important characteristic features of the modular protein are completely conserved; in particular, all cysteine residues align perfectly. The avian LRP/alpha 2MR is post-translationally cleaved in the same fashion as its human counterpart, and the resulting 515-kDa extracellular subunit binds Ca2+, alpha 2-macroglobulin, and vitellogenin. The results indicate that avian LRP/alpha 2MR genes have emerged from an ancestor designed to ensure a pivotal event in the reproduction of oviparous species, i.e. vitellogenesis, and that mammalian LRP/alpha 2MRs have acquired features required for functioning in plasma clearance of certain non-yolk proteins.