Interference of amino‐terminal desmin fragments with desmin filament formation

Short polypeptides from intermediate filament (IF) proteins containing one of the two IF-consensus motifs interfere severely with filament assembly in vitro. We now have systematically investigated a series of larger fragments of the muscle-specific IF protein desmin representing entire functional domains such as coil I or coil 2. "Half molecules" comprising the amino-terminal portion of desmin, such as Des Delta C240 and the "tagged" derivative Des(ESA)Delta C244, assembled into large, roundish aggregates already at low ionic strength, Des Delta C250 formed extended, relatively uniform filaments, whereas Des Delta C265 and Des Delta C300 were soluble under these conditions. Surprisingly, all mutant desmin fragments assembled very rapidly into long thick filaments or spacious aggregates when the ionic strength was raised to standard assembly conditions. In contrast, when these desmin mutants were assembled in the presence of wild-type (WT) desmin, their assembly properties were completely changed: The elongation of the two shorter desmin fragments was completely inhibited by WT desmin, whereas Des Delta C250, Des Delta C265 and Des Delta C300 coassembled with desmin into filaments, but these mixed filaments were distinctly disturbed and exhibited a very different phenotype for each mutant. After transfection into fibroblasts and cardiomyocytes, the truncated mutant Des(ESA)Delta C244 localized largely to the cytoplasm, as revealed by a tag-specific monoclonal antibody, and also partially colocalized there with the collapsed endogenous vimentin and desmin systems indicating its interference with IF-organizing processes. In contrast, in cells without an authentic cytoplasmic IF system such as line SW13, Des(ESA)Delta C242 entered the nucleus and was deposited in small dot-like structures in chromatin-free spaces without any noticeable effect on nuclear morphology.