A hyaluronan binding link protein gene family whose members are physically linked adjacent to chondroitin sulfate proteoglycan core protein genes: the missing links

We describe a vertebrate hyaluronan and proteoglycan binding link protein gene family (HAPLN), consisting of four members including cartilage link protein. The encoded proteins share 45-52% overall amino acid identity. In contrast to the average sequence identity between family members, the sequence conservation between vertebrate species was very high. Human and mouse link proteins share 81-96% amino acid sequence identity. Two of the four link protein genes (HAPLN2 and HAPLN4) were restricted in expression to the brain/central nervous system, while one of the four genes (HAPLN3) was widely expressed. Genomic structures revealed that all four HAPLN genes were similar in exon-intron organization and were also similar in genomic organization to the 5' exons for the CSPG core protein genes. Strikingly, all four HAPLN genes were located immediately adjacent to the four CSPG core protein genes creating four pairs of CSPG-HAPLN genes within the mammalian genome. Furthermore, the two brain-specific HAPLN genes (HAPLN2 and HAPLN4) were physically linked to the brain-specific CSPG genes encoding brevican and neurocan, respectively. The tight physical association of the HAPLN and CSPG genes supports a hypothesis that the first HAPLN gene arose as a partial gene duplication event from an ancestral CSPG gene. There is some degree of coordinated expression of each gene pair. Collectively, the four HAPLN genes are expressed by most tissue types, reflecting the fundamental importance of the hyaluronan-dependent extracellular matrix to tissue architecture and function in vertebrate species. Comparison of the genomic structures for the HAPLN, CSPG genes and other members of the link module superfamily provide strong support for a common evolutionary origin from an ancestral gene containing one link module encoding exon.