ROLES OF HEAD AND TAIL DOMAINS IN ALPHA-INTERNEXINS SELF-ASSEMBLY ANDCOASSEMBLY WITH THE NEUROFILAMENT TRIPLET PROTEINS

The roles of the head and tail domains of alpha-internexin, a type IV neuronal intermediate filament protein, in its self-assembly and coass emblies with neurofilament triplet proteins, were examined by transien t transfections with deletion mutants in a non-neuronal cell line lack ing an endogenous cytoplasmic intermediate filament network, The resul ts from the self-assembly studies showed that the head domain was esse ntial for alpha-internexin's ability to self-assemble into a filament network and the tail domain was important for establishing a proper fi lament network. The data from the coassembly studies demonstrated that alpha-internexin interacted differentially with the neurofilament tri plet protein subunits, Wild-type NF-L or NF-M, but not NF-H, was able to complement and form a normal filament network with the tailless alp ha-internexin mutant, the alpha-internexin head-deletion mutant, or th e alpha-internexin mutant missing the entire tail and some amino-termi nal portion of the head domain, In contrast, neither the tailless NF-L mutant nor the NF-L head-deletion mutant was able to form a normal fi lament network with any of these alpha-internexin deletion mutants, Ho wever, coassembly of the tailless NF-M mutant with the alpha-internexi n head-deletion mutant and coassembly of the NF-M head-deletion mutant with the tailless alpha-internexin mutant resulted in the formation o f a normal filament network, Thus, the coassembly between alpha-intern exin and NF-M exhibits some unique characteristics previously not obse rved with other intermediate filament proteins: only one intact tail a nd one intact head are required for the formation of a normal filament network, and they can be present within the same partner or separatel y in two partners.