Network incorporation of intermediate filament molecules differs between preexisting and newly assembling filaments

When studying the way in which intermediate filaments assemble in vivo, it is important to distinguish between the incorporation of intermediate filament proteins into an existing intermediate filament network and the ability to form a new network within cells. To distinguish between these alternatives, we have made a hybrid construct consisting of the rod and tail domains of murine glial fibrillary acidic protein (GFAP) coupled to the head domain of bovine keratin 19, called K19GFAP. The assembly characteristics of K19GFAP were analyzed in vitro and in vivo. Replacement of the head domain with the bovine K19 sequence did not prevent the incorporation of K19GFAP into the existing network of vimentin intermediate filaments in NIH 3T3 cells but it was incompatible with de novo formation of filament networks in the epithelial cell line MCF-7, which lacks an endogenous vimentin network. By in vitro assembly studies, it was confirmed that K19GFAP was unable to assemble into typical intermediate filaments. We also investigated the ability of an appropriate type II keratin partner to rescue K19GFAP from incorporation into a vimentin network and initiate de novo filament assembly, using the fibroblast cell line KF-K8(3), an NIH 3T3 fibroblast cell line expressing a single human keratin, K8. The results confirm the importance of the coiled coil interactions in determining the fate of intermediate filament proteins. The results also emphasize that filament networks can not only tolerate but also incorporate assembly-deficient intermediate filament protein subunits.