Associates with the organic matrix of calcified cartilage. Expressed in heart, calcified cartilage (branchial arches), vertebra, gills and kidney. Within the gills, MGP is found mainly in the proliferating chondrocytes and the pericellular cartilage matrix in the incipient calcified cartilage of branchial arches and filaments. Requires vitamin K-dependent gamma-carboxylation for its function. Belongs to the osteocalcin/matrix Gla protein family.