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chapter
21
Muscle and Nonmuscle Contractile Systems
synthesis. There is typically about 90% homology between
the various tubulins for both the
a-
and /
1
-forms, and the
tubulins are functionally quite alike, but some additional
diversity can be created by posttranslational modification.
Removal of the C-terminal tyrosine from a-tubulin by a
specific carboxypeptidase (detyrosinase) affects primarily
a-tubulin that is incorporated into a microtubule. Tyrosine
can be reattached by a second enzyme, tubulintyrosine
ligase, which acts preferentially on monomeric a-tubulin
in a reaction that requires hydrolysis of ATR Tyrosinated
a-tubulin is found in all microtubules, while detyrosinated
a-tubulin is found preferentially in the more stable tubular
structures such as cilia. In some organisms, acetylation of
a lysine in a-tubulin occurs during or just after incorpora-
tion into the microtubule. Acetylation has no known effect
on the behavior of microtubules in which it occurs. Some
serine residues in /3-tubulin can be phosphorylated, but
again the significance is unknown. Oxidation and block-
ing of sulfhydryl groups can prevent tubule assembly and
may promote tubule disassembly.
Tubulin is similar to actin and myosin in its ability to
spontaneously polymerize
in vitro.
Like actin, there is a
critical concentration, Cc, of a/3 dimers at which equilib-
rium between association and dissociation occurs, and the
microtubule, like actin, has a polarity and undergoes poly-
merization and depolymerization more rapidly at one end
[(+) end] than at the other [(—) end]. The heterodimers
polymerize a to ^ all along the protomer, so that the pro-
tomer has a polarity,
a
at one end, /3 at the other, and all of
the protomers associate with the same polarity. So one end
of a microtubule is ringed with a-tubulin and the other with
/3-tubulin. The a/3 heterodimers each bind two molecules
of GTP. The a-tubulin site binds GTP irreversibly and
does not hydrolyze it because /3-tubulin shields the site
from water, but the /3-tubulin site binds GTP reversibly
and hydrolyzes it to GDP after incorporation into the pro-
tomer, and especially after additional dimers have been
added. This site is called the exchangeable site because
GTP can displace GDP from it. Microtubules, then, tend
to be capped with GTP dimers, but may be capped with
GDP dimers if the microtubule has shortened, exposing
GDP-tubulin, or when the microtubule has not grown for
such a long time that most GTP has hydrolyzed. When
the terminal dimers at the (+) end of the microtubule have
GTP at the exchangeable site, the structure grows at dimer
concentrations above Cc and slowly depolymerizes below
Cc. When the (+) end dimers have GDP, however, depoly-
merization is rapid below Cc. This raises the surrounding
dimer concentration and accelerates the growth of those
microtubules that are elongating. Thus, microtubules are
inherently unstable unless restrained in a structure like
cilia by numerous accessory proteins.
Formation and stabilization of microtubules other than
in cilia and flagella usually involves a group of pro-
teins called microtubule-associated proteins (MAPs). One
class, called assembly MAPs, cross-link microtubules in
the cytosol. These have two large domains, a basic micro-
tubule binding domain and an acidic projection domain,
so named because it is a filamentous structure project-
ing from the microtubule. The projection domain binds
to membrane proteins, intermediate filaments, or other
microtubules. In the latter case its length determines the
spacing between microtubules. MAPs are divided into
two groups. Type I MAPs—MAPI A and MAP IB—are
large proteins found mainly in neurons but also in other
cell types. Each is derived from a larger precursor which
is proteolytically processed to yield one light chain and
one heavy chain. Type II MAPs include MAP2, MAP4,
and tau protein. These are smaller than type 1, and are
characterized by three or four repeats of an 18-amino-
acid sequence in the microtubule binding region. MAP2
proteins are found only in dendrites where they connect
microtubules to each other and to cytoskeletal IFs. The
tau proteins are a group of four or more proteins derived
from the same gene by alternative mRNA splicing. They
accelerate polymerization and cross-link microtubules.
They may play a role in stabilizing the long and quite
permanent microtubules found in axons.
MAPs generally inhibit depolymerization, and some en-
hance polymerization. Several MAPs can be phosphory-
lated on their projection domains by mitogen-activated
protein kinase, or MAP kinase, which prevents them from
binding microtubules. There is some evidence that MAPs
can also be phosphorylated by Ca-CaMKII and that they
may be directly influenced by Ca-CaM. Thus, MAPs may
be modulated by signal pathways using Ca2+ or MAP
kinase, although specific roles for such control have not
been clearly demonstrated.
Dynein
is the motor protein in cilia and flagella. Dynein
arms are arranged in two groups called inner and outer
dynein arms, spaced at 24-nm intervals along the A mi-
crotubule. Dynein is a large family of microtubule-based
motor proteins. Axonemal dyneins are multimers of (very)
heavy chains, light chains, and intermediate chains. Each
heavy chain has a large globular region with two stalks
extending from it. One stalk connects to a cluster of pro-
teins forming a “base” that attaches to the A microtubule.
The other stalk projects 10 nm toward the B microtubule
and forms a small head that binds to it. The heavy chain
is quite large, about 4500 amino acids long with a molec-
ular weight of more than 540,000. It has ATPase activity,
especially when bound to the B microtubule, which is be-
lieved to be localized to the head, near the tubulin binding
site. Most of the smaller chains are clustered at the base
