| The carboxyl-terminal globular domain of human aggrecan has been shown previously to contain an alternatively spliced epidermal growth factor (EGF)-like domain, but aggrecans of other species seemingly lack this module. Therefore, the studies summarized in this dissertation have employed the sensitive reverse transcription/polymerase chain reaction method on cartilage-derived RNAs to explore the possible 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 (EGF2) was detected in human aggrecan, establishing the presence of two of these domains. Highly homologous modules are present in aggrecans of other species. Transcripts that code for the EGF2 domain comprise 4-8% of the total aggrecan mRNA. In contrast, the previously described EGF-like module (EGF1) is expressed in a species-specific manner and shows high sequence variation among the species. The EGF1 sequence is present in approximately one-fourth of the human aggrecan mRNAs. On the other hand, this sequence is expressed in only a minor fraction of the bovine aggrecan transcripts (;Both EGF1 and EGF2 domains are coded by separate exons in human and bovine aggrecan genes, raising the total exon number to 19. Both exons are alternatively spliced and expressed independently, i.e. four different transcripts are present in human chondrocytes. The intron sequences upstream to the EGF1 exon indicate that an impaired polypyrimidine tract may be responsible for the species-specific splicing of this exon.;Although the function of the two EGF-like domains is not understood, the uniform expression and high conservation of the EGF2 domain may suggest a general role for this module. In contrast, the high evolutional variability and species-specific expression of the EGF1 exon may imply a loss-of-function module. |
