Homologues of vertebrate type I, II and III intermediate filament (IF) proteins in an invertebrate: the IF multigene family of the cephalochordate Branchiostoma

Tissue-specific co-expression and in vitro heteropolymer formation of the two small Branchiostoma intermediate filament proteins A3 and B2

“…These properties indicate some type III character for the B1 protein. 12,13 Here, we show that A3 and B2, which are co-expressed in the intestinal epithelial cells, form obligatory heteropolymeric IF based on a hetero coiled coil consisting of one A3 and one B2 polypeptide chain.…”

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

“…This situation generally reflects the difficulty to clearly classify IF proteins exclusively on the basis of their gene structures and/or amino acid sequences, especially if evolutionary "distant" members of this very complex multigene family are considered. For example, most keratins in lancelets also are encoded by genes that show structures typical for type III IF proteins and could only be identified by inclusion of filament assembly and comprehensive expression studies [24,25,28]. The genes of the keratins identified in sea squirts even show different structures, but each does at least share 4 intron positions common in vertebrate type I-III IF protein genes [32].…”

“…Thus, from the known variety of vertebrate keratins, K8 and K18 seem to be the most ancient examples. IF proteins showing several structural and functional similarities to vertebrate keratins have also been detected in lower chordates such as sea squirts and lancelets [23][24][25][26][27][28][29][30][31][32]. However, solely on the basis of their amino acid sequences, these "keratin-like" IF proteins found in lower chordates cannot clearly be classified as type I or type II IF proteins and the organization of their genes either corresponds to the structure typical for type III IF proteins (lancelets) or even shows different structures (sea squirts [32]).…”

Apoptosis and keratin intermediate filaments

“…While keratins are promiscuous in their ability to heterpolymerize from different type I and II proteins in vitro, 23,24 preferential assembly of certain keratin pairs in vivo has been revealed by gene knockout experiments. Thus the removal of K18 results in the loss of K7 as well as K8 protein in intestine.…”