Practice question

Intermediate filament protein family comprises over 70 members classified by sequence homology and expression pattern regulated developmentally. Signature central rod domain about 310 amino acids alpha helical coiled coil flanked by variable head tail domains mediating assembly and interactions. Major subgroups include type I acidic keratins 28 genes and type II basic keratins 26 genes obligate heteropolymers in epithelia forming extensive cytoskeleton; type III includes vimentin mesenchymal fibroblasts endothelial desmin skeletal cardiac smooth muscle GFAP astrocytes peripherin peripheral neurons; type IV neurofilaments NF light medium heavy in central neurons determining axon caliber; type V lamins nuclear envelope; type VI nestin progenitor cells. Keratin represents classic component forming dense tonofilament networks resisting mechanical stress chemical insult and apoptosis. Tubulin alpha beta heterodimers make microtubules actin forms thin filaments myosin motor domain binds actin not intermediate. Therefore keratin is recognized intermediate filament constituent essential for epithelial integrity mutations causing epidermolysis bullosa simplex due to cell fragility under friction trauma.