Target Discovery and Validation Reviews and Protocols

Keratin Transgenics and Knockouts 205
Fig. 1. Schematic alignment of both mouse and human keratin type I and type II gene
cluster. One black arrowhead represents approximately two to three keratin genes orientated
in the direction of the tip. Gray arrowheads in (A) delineate keratins of the inner
root sheet of hair follicles (6). The light gray arrowhead in (B) indicates an array of
processed pseudogenes unrelated to keratins. The dark gray arrowheads in (B) highlight
the only type I keratin (K18) at the end of both mouse and human type II cluster. KAP’s
is the abbreviation of a cluster composed of keratin associated proteins (see Note 3).
“rod” domain flanked by non-α helical amino-terminal and carboxy-terminal
domains, which guide two monomers into a double-stranded and parallel coiledcoil
formation. The rod domain itself is composed of four highly conserved
domains, termed 1A and B and 2A and B connected by non-α-helical linkers
(see Fig. 2). An individual α-helical segment exhibits a heptad substructure
(abcdefg) n , in which positions a and d are occupied by hydrophobic amino acids,
thus establishing a seam around the axis of a single right-handed α-helix that
contributes to the formation of the coiled-coil structure. Only a short sequence in
the middle of segment 2B interrupts this regular phasing of heptades that does not
form a coiled-coil structure but rather runs in parallel with the corresponding part
of the second molecule of the adjacent coiled-coil dimmer (8,9).
Special motifs within the 1B and 2B coil might trigger the self-assembly
of an obligatory type I and II keratin into a parallel heterodimer that further
assembles into antiparallel, half-staggered tetramers that laterally associate to
the so-called unit-length filaments (ULFs). Finally, these structures anneal longitudinally
into nonpolar filaments compacting to IFs of approx 11 nm in diameter
(10,11). There is mounting evidence from in vitro as well from in vivo
studies that at least some keratins might replace each other, supporting the idea
of functional redundancy (8).
Motifs within the head domain have been shown to be essential for the formation
of filaments, whereas tail sequences are suspected to regulate filament
diameter or to interact with associated proteins. However, a side contribution of
the tail to filament bundling, at least in some keratins, cannot be ruled out (12).