Keratin intermediate filament chains in tuatara (Sphenodon punctatus): A comparison of tuatara and human sequences

Determination of the sequences of the keratin intermediate filament chains in tuatara has shown that these are closely akin to the a-keratins in human and other vertebrates, especially in the central, coiled-coil rod region. The domain lengths within the rod are preserved exactly, both Type I and Type II chains have been recognised, and sequence identity/homology exists between their respective chains. Nonetheless, there are characteristic differences in amino acid composition and sequence between their respective head (N-terminal) domains and their tail (C-terminal) domains, though some similarities are retained. Further, there is evidence of tandem repeats of a variety of lengths in the tuatara heads and tails indicative of sequence duplication events. These are not evident in human a-keratins and would therefore have implications for the physical attributes of the tissues in the two species. Multiple families of keratin-associated proteins that are ultra-high cysteine-rich or glycine + tyrosinerich in human and other species do not have direct equivalents in the tuatara. Although high-sulphur proteins are present in tuatara the cysteine residue contents are significantly lower than in human. Further, no sequence homologies between the HS proteins in the two species have been found, thereby casting considerable doubt as to whether any matrix-forming high-sulphur proteins exist in tuatara. These observations may be correlated with the numerous cysteine-rich beta-keratins (corneous beta-proteins) that are present in tuatara, but which are conspicuously absent in mammals.

Automated summary

The keratin intermediate filament chains in tuatara are similar to a-keratins in human and other vertebrates, especially in the central rod region. However, there are significant differences in amino acid composition and sequence between their head and tail domains. Additionally, there is evidence of sequence duplication events in tuatara, but high-sulphur proteins in tuatara have lower cysteine residue contents compared to humans. The presence of cysteine-rich beta-keratins in tuatara, which are absent in mammals, further distinguishes the two species.