Background: Intermediate Filaments (IFs) are major constituents of the cytoskeletal
systems in animal cells.
Objective: To gain insights into the structure-function relationship of invertebrate cytoplasmic
IF proteins, we characterized an IF protein from the platyhelminth, Dugesia japonica, termed
Methods: cDNA cloning, in situ hybridization, immunohistochemical analysis, and IF assembly
experiments in vitro using recombinant Dif-1, were performed for protein characterization.
Results: The structure deduced from the cDNA sequence showed that Djf-1 comprises 568 amino
acids and has a tripartite domain structure (N-terminal head, central rod, and C-terminal tail)
that is characteristic of IF proteins. Similar to nuclear IF lamins, Djf-1 contains an extra 42 residues in
the coil 1b subdomain of the rod domain that is absent from vertebrate cytoplasmic IF proteins and
a nuclear lamin-homology segment of approximately 105 residues in the tail domain;
however, it contains no nuclear localization signal. In situ hybridization analysis showed that
Djf-1 mRNA is specifically expressed in cells located within the marginal region encircling the
worm body. Immunohistochemical analysis showed that Djf-1 protein forms cytoplasmic IFs
located close to the microvilli of the cells. In vitro IF assembly experiments using recombinant
proteins showed that Djf-1 alone polymerizes into IFs. Deletion of the extra 42 residues in the
coil 1b subdomain resulted in the failure of IF formation.
Conclusion: Together with data from other histological studies, our results suggest that Djf- 1 is
expressed specifically in anchor cells within the glandular adhesive organs of the worm and that
Djf-1 IFs may play a role in protecting the cells from mechanical stress.