We have identified a transmembrane collagen, collagen XXIII, in rat prostate carcinoma cells. Differential display of mRNA expression in prostate carcinoma sublines with varying metastatic potential revealed overexpression of this transcript in the metastatic AT6.1 subline. cDNA cloning identified a 2733-bp transcript from AT6.1 RNA, encoding a protein of 532 amino acids, together with a 3067-bp human homologue, resulting in a 540-amino acid protein. Collagen XXIII is predicted to be a type II membrane protein consisting of an amino-terminal cytoplasmic domain, a transmembrane region, and three collagenous domains flanked by short noncollagenous domains. Collagen XXIII is a new member of the transmembrane collagen family, showing structural homology with the transmembrane collagens XIII and XXV. We present evidence that collagen XXIII is expressed as a approximately 75-kDa protein at the cell surface and that it can be cleaved by furin protease activity. Cleavage results in a approximately 60-kDa soluble protein that forms a multimeric complex and exhibits a low affinity interaction with heparin.