384
– the MTs are hollow cylinders 23 nm in diameter, most commonly comprised of
13 protofi laments. These in turn are polymers of alpha- and beta-tubulin, and
play key roles in intracellular transport and the formation of the mitotic spindle”
(Norlén et al. 2007 ; see also Glotzer 2009 ; Karantza 2011 ; Steinert et al. 1993 ).
IFs differ from MTs and actin fi laments due to that they are encoded by a large
family of genes expressed in a tissue- and differentiation state-specifi c manner.
Their rod domain amino-acid sequence is used as the key parameter for their classifi cation. Thus, according to Human Intermediate Filament Mutation Database,
www.interfi l.org ), there are 70 intermediate fi lament genes in six distinct classes in
human alone. “Considering that many intermediate fi lament proteins can copolymerize with several different partners, the number of unique 10 nm intermediate
fi laments in humans is likely in the hundreds. The diversity of intermediate fi laments in other species is not as well-described, but is undoubtedly impressive given
the diversity in humans,” (Fudge et al. 2009b ). Here, some examples of the Ifs
diversity represented in the paper by Herrmann et al. ( 2007 ) as follow:
“Types 1 and 2 IFs are found primarily in epithelial cells and include the acidic
and basic keratins, respectively. Type 3 IFs include vimentin, desmin and glial
fi brillary acidic protein; type 4 IFs assemble into neurofi laments; and type 5 IFs are
the nuclear lamins,” (Karantza 2011 ; see also Herrmann et al. 2007 ).
The assembly/disassembly cycle of keratin IF (Kölsch et al. 2010 ) allows rapid
network remodeling within epithelial cells without network disruption. In brief,
“the cycle begins with nucleation of keratin particles at the cell periphery, often in
close proximity to lamellipodial focal adhesions (Fig. 11.2 ). This is followed by
elongation of newly formed keratin particles during actin-dependent translocation
toward the peripheral keratin network. After integration of precursor particles into
the network, keratins IFs continue to move toward the nucleus and bundle. Some of
them disassemble into soluble oligomers that rapidly diffuse throughout the cytoplasm and are available for another round of nucleation in the cell periphery. Others
mature into a stable network that surrounds the nucleus, and is anchored to desmosomes
and hemidesmosomes. Collectively, cycling allows the epithelial cytoskeleton to
remain in motion without loss of structural integrity,” (Windoffer et al. 2011 ).
How keratins provide rigidity and strength but at the same time remain dynamic
and fl exible is one of the key questions (Windoffer et al. 2011 ). Therefore, the biomechanical properties of intermediate fi laments are intensively studied (see for
review Ma et al. 1999 ; Bousquet et al. 2001 ; Kreplak et al. 2002 ; Russell et al. 2004 ;
Kreplak and Fudge 2007 ; Wagner et al. 2007 ; Flitney et al. 2009 ; Leitner et al.
2012 ). The mechanical function of intermediate fi laments is evident in many biological systems. For example, “the high storage modulus of the keratin IF network,
coupled with its solid-like rheological behavior, supports the role of keratin IF as an
intracellular structural scaffold that helps epithelial cells to withstand external
mechanical forces” (Sivaramakrishnan et al. 2008 ).
Furthermore, in the work by Wagner and co-workers ( 2007 ), we can fi nd following statement:
“Intermediate fi laments, like their cytoskeletal counterpart F-actin, are examples
of semi-fl exible polymers. This is a class of polymers that are too soft to be truly
11 Marine Keratins
– the MTs are hollow cylinders 23 nm in diameter, most commonly comprised of
13 protofi laments. These in turn are polymers of alpha- and beta-tubulin, and
play key roles in intracellular transport and the formation of the mitotic spindle”
(Norlén et al. 2007 ; see also Glotzer 2009 ; Karantza 2011 ; Steinert et al. 1993 ).
IFs differ from MTs and actin fi laments due to that they are encoded by a large
family of genes expressed in a tissue- and differentiation state-specifi c manner.
Their rod domain amino-acid sequence is used as the key parameter for their classifi cation. Thus, according to Human Intermediate Filament Mutation Database,
www.interfi l.org ), there are 70 intermediate fi lament genes in six distinct classes in
human alone. “Considering that many intermediate fi lament proteins can copolymerize with several different partners, the number of unique 10 nm intermediate
fi laments in humans is likely in the hundreds. The diversity of intermediate fi laments in other species is not as well-described, but is undoubtedly impressive given
the diversity in humans,” (Fudge et al. 2009b ). Here, some examples of the Ifs
diversity represented in the paper by Herrmann et al. ( 2007 ) as follow:
“Types 1 and 2 IFs are found primarily in epithelial cells and include the acidic
and basic keratins, respectively. Type 3 IFs include vimentin, desmin and glial
fi brillary acidic protein; type 4 IFs assemble into neurofi laments; and type 5 IFs are
the nuclear lamins,” (Karantza 2011 ; see also Herrmann et al. 2007 ).
The assembly/disassembly cycle of keratin IF (Kölsch et al. 2010 ) allows rapid
network remodeling within epithelial cells without network disruption. In brief,
“the cycle begins with nucleation of keratin particles at the cell periphery, often in
close proximity to lamellipodial focal adhesions (Fig. 11.2 ). This is followed by
elongation of newly formed keratin particles during actin-dependent translocation
toward the peripheral keratin network. After integration of precursor particles into
the network, keratins IFs continue to move toward the nucleus and bundle. Some of
them disassemble into soluble oligomers that rapidly diffuse throughout the cytoplasm and are available for another round of nucleation in the cell periphery. Others
mature into a stable network that surrounds the nucleus, and is anchored to desmosomes
and hemidesmosomes. Collectively, cycling allows the epithelial cytoskeleton to
remain in motion without loss of structural integrity,” (Windoffer et al. 2011 ).
How keratins provide rigidity and strength but at the same time remain dynamic
and fl exible is one of the key questions (Windoffer et al. 2011 ). Therefore, the biomechanical properties of intermediate fi laments are intensively studied (see for
review Ma et al. 1999 ; Bousquet et al. 2001 ; Kreplak et al. 2002 ; Russell et al. 2004 ;
Kreplak and Fudge 2007 ; Wagner et al. 2007 ; Flitney et al. 2009 ; Leitner et al.
2012 ). The mechanical function of intermediate fi laments is evident in many biological systems. For example, “the high storage modulus of the keratin IF network,
coupled with its solid-like rheological behavior, supports the role of keratin IF as an
intracellular structural scaffold that helps epithelial cells to withstand external
mechanical forces” (Sivaramakrishnan et al. 2008 ).
Furthermore, in the work by Wagner and co-workers ( 2007 ), we can fi nd following statement:
“Intermediate fi laments, like their cytoskeletal counterpart F-actin, are examples
of semi-fl exible polymers. This is a class of polymers that are too soft to be truly
11 Marine Keratins