Target Discovery and Validation Reviews and Protocols

206 Vijayaraj, Söhl, and Magin
Fig. 2. (A) General structure of an intermediate filament (IF) protein divided into
three different sections: coiled-coil rod domain (green); head and tail domain (yellow);
keratin type I cluster genes (B, above). The latter are very similar to keratin type II cluster
genes (below) except that they are missing a traditional head and tail domain.
Vertical bars indicate the frequency of already detected human pathological mutations
at a certain position.
Mutations identified in the tail sequences of K1 and K5 have been shown to
be responsible for ichthyosis hystrix and epidermolysis bullosa simplex (EBS)
with migrating erythema. However, the most severe mutations found in keratin
genes are located in the evolutionarily conserved end domains of the α helical
rod (13) rather than in both head and tail domains (14,15). Generally, point
mutations, such as those found in K5 and K14 that cause different forms of EBS
(16,17) (see Fig. 3A), provide insight into the process of IF formation.
Interestingly, the K14R125C/H mutation has no effect on filament assembly or
length in vitro (10), but it reveals a decreased resilience of filaments against
large deformations (18) and leads to filament aggregation depending on the
molar ratio of mutant to the wild-type protein (19). Filament formation, per se,
seems to initiate close to the plasma membrane. The role of desmosomes, which
might play a role in IF assembly is still unclear (20).
Keratin-deficient animals helped answer the question of whether there is a
functionally redundant keratin substituting an ablated keratin. The K5 null mice
(see Fig. 3F) die shortly after birth, lack keratin filaments in the basal epidermis,
and are more severely affected than K14(–/–) mice. However, the unaffected
formation of intact K1- and K10-containing IFs in the suprabasal layers
indicates that K5 is not needed as a “chaperone” or “scaffold” for K1 and K10
(21). Additionally, many studies have been undertaken to determine the influence
of posttranslational modifications such as phosphorylation, glycosylation,
transglutamination, deimination, and ubiquitination on keratin assembly and