The kidney injury-molecule-1 (“KIM-1”) gene was identified as a gene whose expression is upregulated in post-ischemic rat kidney cells as compared to the expression of the gene in non-injured rat kidney cells. The KIM-1 gene encodes a type I cell membrane glycoprotein. Two forms of the gene have been described in humans. One form is named KIM-1(a) and is 334 amino acids in length. The second form is named KIM-1(b) and is 359 amino acids in length. The two human homologues are identical throughout their 323 amino terminal amino acid sequences but differ in sequence in their carboxy terminal amino acids. The KIM-1 gene is expressed in dedifferentiated proximal tubular epithelial cells in damaged regions. High level expression is observed in the S3 segment of the proximal tubule in the outer stripe of the outer medulla. This region is highly susceptible to damage as a result of ischemia or toxins.
The amino terminal region of the KIM-1 protein includes the extracellular portion of the KIM-1 protein. This region includes a six-cysteine immunoglobulin-like domain and a T/SP rich domain characteristic of mucin-like O-glycosylated proteins. Immunoglobulin-like domains have been widely implicated in mediating protein-protein interactions, particularly at the cell surface where they are responsible for cell-cell and cell-extracellular matrix interactions.