Collectrin (also Transmembrane protein 27) is a 37-43 kDa member of the MEM27 family of proteins. It is expressed on pancreatic beta-cells where it regulates insulin secretion, and on renal tubular epithelium where it mediates amino acid transport. Mature mouse Collectrin is a 208 amino acid (aa) type I transmembrane glycoptotein. It contains a 127 aa extracellular region (aa 15-141) and a 60 aa cytoplasmic domain (aa 163-222). Collectrin dimerizes on the cell surface, and undergoes proteolytic cleavage to generate a 26 kDa soluble fragment. Over aa 15-144, mouse Collectrin shares 92% and 86% aa identity with rat and human Collectrin, respectively.