We completed the cDNA cloning and sequencing of gp330, the major kidney glomerular antigen for rat Heymann nephritis. The deduced 4660-aa sequence, expected to constitute a mature protein of M(r) 516,715, consists of a probable N-terminal signal peptide sequence (25 aa), an extracellular region (4400 aa), a single transmembrane domain (22 aa), and a C-terminal cytoplasmic tail (213 aa). The extracellular region contains three types of cysteine-rich repeats characteristic of the low density lipoprotein receptor (LDLR) gene family--36 LDLR ligand-binding repeats forming four clusters of putative ligand-binding domains, 16 growth factor repeats separated by 8 YWTD spacer regions, and 1 C-terminal epidermal growth factor repeat. The cytoplasmic tail contains two copies of the (FX)NPXY motif, which represents a signal for coated pitmediated internalization and an additional similar motif. The overall structure of gp330 is similar to that of the LDLR-related protein (LRP)/alpha 2-macroglobulin receptor and shows even greater similarity to the Caenorhabditis elegans protein, reported as a homologue of LRP. However, gp330 differs from these proteins in (i) the cysteine-rich repeat arrangements found in the extreme extracellular N- and C-terminal regions, (ii) the distribution pattern of cysteine residues in the YWTD spacer regions, (iii) the location of the RX(K/R)R consensus recognition sequence of furin, a precursor processing endoprotease, and (iv) the length and structure of the cytoplasmic tail. We suggest the name megalin (from Greek mega) for gp330, the largest plasma membrane protein identified so far in vertebrates. The cloned cDNA will be useful for studies on the physiological functions of gp330/megalin and for determining its role in Heymann nephritis.