Abstract
BackgroundThe expression of intermediate filaments (IFs) is a hallmark feature of metazoan cells. IFs play a central role in cell organization and function, acting mainly as structural stress-absorbing elements. There is growing evidence to suggest that these cytoskeletal elements are also involved in the integration of signalling networks. According to their fundamental functions, IFs show a widespread phylogenetic expression, from simple diblastic animals up to mammals, and their constituent proteins share the same molecular organization in all species so far analysed. Arthropods represent a major exception in this scenario. Only lamins, the nuclear IF proteins, have so far been identified in the model organisms analysed; on this basis, it has been considered that arthropods do not express cytoplasmic IFs.ResultsHere, we report the first evidence for the expression of a cytoplasmic IF protein in an arthropod - the basal hexapod Isotomurus maculatus. This new protein, we named it isomin, is a component of the intestinal terminal web and shares with IFs typical biochemical properties, molecular features and reassembly capability. Sequence analysis indicates that isomin is mostly related to the Intermediate Filament protein C (IFC) subfamily of Caenorhabditis elegans IF proteins, which are molecular constituents of the nematode intestinal terminal web. This finding is coherent with, and provides further support to, the most recent phylogenetic views of arthropod ancestry. Interestingly, the coil 1a domain of isomin appears to have been influenced by a substantial molecular drift and only the aminoterminal part of this domain, containing the so-called helix initiation motif, has been conserved.ConclusionsOur results set a new basis for the analysis of IF protein evolution during arthropod phylogeny. In the light of this new information, the statement that the arthropod phylum lacks cytoplasmic IFs is no longer tenable.See commentary article: http://www.biomedcentral.com/1741-7007-9-16.
Highlights
The expression of intermediate filaments (IFs) is a hallmark feature of metazoan cells
For cytoplasmic IF proteins, phylogenetic surveys have shown evidence of the occurrence of two molecular prototypes, which segregate according to phylogenetic lineages: the L-type, which shares with lamins a longer rod domain and is expressed in protostome phyla; and the S-type, endowed with a shorter rod domain, which is thought to be arisen from the L-type by a deletion event and, until now, has only been detected in the three deuterostome chordate phyla [6]
We have preliminarily reported that the web does not consist of actin, since it is not decorated by specific antibodies or by heavy meromyosin fragments and does not disassemble after treatment with cytochalasin B [15]
Summary
The expression of intermediate filaments (IFs) is a hallmark feature of metazoan cells. For cytoplasmic IF proteins, phylogenetic surveys have shown evidence of the occurrence of two molecular prototypes, which segregate according to phylogenetic lineages: the L-type, which shares with lamins a longer rod domain and is expressed in protostome phyla; and the S-type, endowed with a shorter rod domain, which is thought to be arisen from the L-type by a deletion event and, until now, has only been detected in the three deuterostome chordate phyla [6] On this basis, it has been speculated that cytoplasmic IFs arose early in evolution from a mutated lamin gene [7]. The complexity of the cytoplasmic IF protein repertoire expressed in different metazoan phyla varies, reaching its maximum in vertebrates which express up to 70 proteins belonging to six distinct IF subfamilies [8]
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