Gene Targeting Studies Begin to Reveal the Function of Neurofilament Proteins

A mong cytoskeletal systems, the intermediate filaments (IFs) 1 are unique because they are composed of different proteins in different tissues. These proteins share a common tripartite structure composed of an NH 2 -terminal head domain, a coiled-coil rod domain, and a COOH terminus of varying length. They are subdivided into six IF classes according to their amino acid sequence similarity (for review see Julien and Grosveld, 1991) and to the intron structure of their genomic sequences. Neurofilaments (NFs) are enriched in neurons, especially in large-caliber axons. They belong to the fourth group of IFs, which comprises three subunit proteins: NF-L (apparent molecular mass z 68 kD), NF-M (idem. z 160 kD), and NF-H (idem. z 200 kD) (Hoffman and Lasek, 1975; Liem et al., 1978). Neurofilaments in nerve axons are composed of a parallel array of 10-nm filaments with frequent crossbridges between NFs or between NFs and microtubules (MTs) or membranous organelles (Hirokawa, 1982) (Fig. 1). In vitro reconstitution studies revealed that NF-L by itself forms a 10-nm core filament; if NF-M or NF-H is added, many thin sidearms project from the core (Hisanaga and Hirokawa, 1988). Although all three proteins have the tripartite structure described above, NF-M and NF-H have long, hypervariable COOH-terminal tails (NF-M 439 amino acids, NF-H 660 amino acids) containing several KSP repeats that are targets of phosphorylation. On the basis of the reconstitution studies and of immunocytochemical studies using specific antibodies (Hirokawa et al., 1984; Balin et al., 1991; Mulligan et al., 1991), it is thought that the head and rod domains of NF-L, NF-M, and NF-H form the filament core, and the tails of NF-M and NF-H project from the core to form crossbridges.