A mouse cDNA encoding a novel member of the von Willebrand factor type A-like module superfamily was cloned. The protein precursor of 956 amino acids consists of a putative signal peptide, two von Willebrand factor type A-like domains connected by 10 epidermal growth factor-like modules, a potential oligomerization domain, and a unique segment, and it contains potential N-glycosylation sites. A sequence similarity search indicated the closest relation to the trimeric cartilage matrix protein (CMP). Since they constitute a novel protein family, we introduce the term matrilin-2 for the new protein, reserving matrilin-1 as an alternative name for CMP. A 3. 9-kilobase matrilin-2 mRNA was detected in a variety of mouse organs, including calvaria, uterus, heart, and brain, as well as fibroblast and osteoblast cell lines. Expressed human and rat cDNA sequence tags indicate a high degree of interspecies conservation. A group of 120-150-kDa bands was, after reduction, recognized specifically with an antiserum against the matrilin-2-glutathione S-transferase fusion protein in media of the matrilin-2-expressing cell lines. Assuming glycosylation, this agrees well with the predicted minimum Mr of the mature protein (104,300). Immunolocalization of matrilin-2 in developing skeletal elements showed reactivity in the perichondrium and the osteoblast layer of trabecular bone. CMP binds both collagen fibrils and aggrecan, and because of the similar structure and complementary expression pattern, matrilin-2 is likely to perform similar functions in the extracellular matrix assembly of other tissues.