Intermediate filament (IF) proteins IFA-1 and IFB-1 represent a basic heteropolymeric IF cytoskeleton of nematodes: A molecular phylogeny of nematode IFs.

Intermediate filaments (IF) belong to major cytoskeletal components of metazoan cells. We have previously determined a tissue specific expression and assembly properties of all eleven cytoplasmic IFs (IFA-1 - IFA-4, IFB-1, IFB-2, IFC-1, IFC-2, IFD-1, IFD-2, IFP-1) in C. elegans and reported an essential function for four (IFA-1, IFA-2, IFA-3 and IFB-1) of them. In this study we continued the characterisation of the IF proteins in C. elegans by searching for in vivo polymerisation partners of the IFA proteins. Using the murine IFA-1 to IFA-3-specific monoclonal Ab MH4 and the immunoprecipitation assay as a tool, we identified the heteropolymeric IFA-1/IFB-1 complexes in the whole nematode protein extract, confirming their existence also in vivo. Moreover, in the present study we also analysed evolutionary aspects of the IF proteins in C. elegans and in nematodes. We found 106 C. elegans IF homologs in different nematode clades. Phylogenetic analyses suggest that all nematode IFs (including the three newly identified IF sequences IFA-5, IFCDP-1 and IFCDP-2) might arose from a AB-type IF ancestor through repeated gene duplications and sequence divergence. Interestingly, the C. elegans IF proteins IFA-1 and IFB-1 represent a heteropolymeric IF cytoskeleton in all investigated nematodes, in contrast to other sequences restricted to the clade III-V (IFA-2, IFA-4), III (IFA-5) and V (IFB-2, IFCDP) taxa, or even to the Caenorhabditis genus (IFA-3, IFC-1 to IFP-1). These analyses provide an insight into the origin of the multiple IFs in nematodes and also represent a basis for further studies of these sequences in nematodes.