Genes coding for intermediate filament proteins closely related to the hagfish “thread keratins (TK)” α and γ also exist in lamprey, teleosts and amphibians

The “thread keratins (TK)” α and γ so far have been considered highly specialized intermediate filament (IF) proteins restricted to hagfish. From lamprey, we now have sequenced five novel IF proteins closely related to TKα and TKγ, respectively. Moreover, we have detected corresponding sequences in EST and genomic databases of teleosts and amphibians. The structure of the TKα genes and the positions of their deduced amino acid sequences in a phylogenetic tree clearly support their classification as type II keratins. The genes encoding TKγ show a structure typical for type III IF proteins, whereas their positions in phylogenetic trees favor a close relationship to the type I keratins. Considering that most keratin-like sequences detected in the lancelet also exhibit a gene structure typical for type III IF proteins, it seems likely that the keratin gene(s) originated from an ancient type III IF protein gene. According to EST analyses, the expression of the thread keratins in teleost fish and amphibians may be particularly restricted to larval stages, which, in conjunction with the observed absence of TKα and TKγ genes in any of the available Amniota databases, indicates a thread keratin function closely related to larval development in an aquatic environment.