Mucin CYS domains are ancient and highly conserved modules that evolved in concert

BackgroundMany large secreted gel-forming mucins contain multiple copies of a “naked” cysteine-enriched domain (CYS domain) that interrupt or are adjacent to mucin domains (Ser/Thr/Pro regions). There is less similarity of homologous CYS sequences between species than there is across CYS repeats within species. These CYS domains are likely implicated in reversible mucin–mucin interactions that play a central role in mucus properties.ResultsThe use of multiple genome sequences in a comparative analysis of CYS sequences identified new mucins containing the CYS domain in various species. Furthermore, analysis showed that this domain is at least 650 million-year old and that CYS sequences evolved under strong selective pressure in concerted fashion in all species examined by two mechanisms that never coexist within a given gene: either multiplication of exons encoding CYS modules or successive intra-exonic duplications.ConclusionThe CYS sequences appear to have undergone concerted evolution with a high selective pressure to conserve cysteine and other amino acid residues. This domain is the best-conserved domain in secreted mucins pointing toward an important common function in many different organisms.