Identification and characterization of a novel extracellular matrix protein nephronectin that is associated with integrin alpha 8 beta 1 in the embryonic kidney

he epithelial-mesenchymal interactions required for kidney organogenesis ar e disrupted in mice lacking the integrin alpha8 beta1. None of this integri n's known ligands, however, appears to account for this phenotype. To ident ify a more relevant ligand, a soluble integrin (alpha8 beta1 heterodimer fu sed to alkaline phosphatase (AP) has been used to probe blots and cDNA libr aries. In newborn mouse kidney extracts, alpha8 beta1-AP detects a novel li gand of 70-90 kD. This protein, named nephronectin, is an extracellular mat rix protein with five EGF-like repeats, a mucin region containing a RGD seq uence, and a COOH-terminal MAM domain. Integrin alpha8 beta1 and several ad ditional RGD-binding integrins bind nephronectin. Nephronectin mRNA is expr essed in the ureteric bud epithelium, whereas alpha8 beta1 is expressed in the metanephric mesenchyme. Nephronectin is localized in the extracellular matrix in the same distribution as the ligand detected by alpha8 beta1-AP a nd forms a complex with alpha8 beta1 in vivo. Thus, these results strongly suggest that nephronectin is a relevant ligand mediating alpha8 beta1 funct ion in the kidney. Nephronectin is expressed at numerous sites outside the kidney, so it may also have wider roles in development. The approaches used here should be generally useful for characterizing the interactions of nov el extracellular matrix proteins identified through genomic sequencing proj ects.