Expression of alternatively spliced epidermal growth factor-like domains in aggrecans of different species. Evidence for a novel module.

The carboxyl-terminal globular domain of human aggrecan has been shown previously to contain an alternatively spliced epidermal growth factor (EGF)-like module. We have used reverse transcription/polymerase chain reactions on cartilage-derived RNAs to investigate the heterogeneity in the EGF-like domain content of aggrecans from five species (mouse, rat, dog, bovine, and human). A novel alternatively spliced EGF-like module was detected in human aggrecan, establishing the presence of two of these domains. Highly homologous domains are present in aggrecans of other species, and the expression of these modules is identical (4-8%). They share significant homology with EGF-like domains of differentiation proteins and coagulation factors and have a putative calcium binding site. In contrast to this novel domain (EGF2), the previously described EGF-like module (EGF1) is expressed at a high level exclusively in human aggrecan. The expression of the two EGF-like domains in human aggrecan appears to be independent. Although the function of these domains is not understood, the uniform expression of the EGF2 domain may indicate a general role of this aggrecan module, while the expression of the EGF1 domain may reflect species specificity.