Alpha-internexin has a homologous central rod domain of approximately 310 amino acid residues that form a highly conserved alpha helical region. The central rod domain is responsible for coiled-coil structure and is flanked by an amino terminal head region and a carboxy terminal tail. This rod domain is also involved in the 10 nm filament assembly structure. The head and tail regions contain segments that are highly homologous to the NF-M’s structure. The head region is highly basic and contains many serine and threonine polymers while the tail region has distinct sequence motifs like a glutamate rich region. The alpha domain is composed of heptad repeats of hydrophobic residues that aid the formation of a coiled coil structure. The structure of Alpha-internexin is highly conserved between rats, mice and humans.
Alpha-internexin can form homopolymers, unlike the heteropolymer the neurofilaments form. This formation suggests that α-internexin and the three neurofilaments form separate filament systems. Not only can alpha-internexin form homopolymers but it form a network of extended filaments in the absence of other intermediate filament proteins and efficiently co-assemble with any type IV or type III subunit, in vitro. In Ching et al., a model of the intermediate filaments assembly is proposed. This model includes the following steps:
- Step 1: in the first step of IF assembly two parallel, unstaggered intermediate filament polypeptides chains form a dimer via their a-helical rod domains; these dimers can be either homodimers or heterodimers.
- Step 2: the dimers may associate laterally to form antiparallel, unstaggered tetramers or antiparallel, staggered tetramers.
- Step 3: the dimers may also associate longitudinally with a short head-to-tail overlap of the a-helical rod domains.
- Step 4: these lateral and longitudinal associations lead to the formation of protofibrils (octamers) and ultimately 10 nm intermediate filaments.
The close connection between the neurofilament triplet proteins and α-internexin is quite obvious. α-internexin is functionally interdependent with the neurofilament triplet proteins. If one genetically deletes NF-M and/or NF-H in mice, the transport and presence, in the axons of the Central Nervous System, of α-internexin will be drastically reduced. Not only are they functionally similar, the turnover rates are also similar among the four proteins.
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