utilizes an actin filament as the track. Two other motor proteins, kinesin and
cytoplasmic dynein [20] utilize a microtubule as the track; the kinesin and cytoplasmic dynein run in the opposite directions. The run length of the organelle on the
microtubule track is longer than that on the actin track, and the velocity of the
transport on microtubule is generally faster than that on actin filament (as in the
example shown in Fig. 7.9c), or in the case of the transport of color pigments in
melanocyte [21, 22]). The transport of the color pigment is remarkable, because
there is a synergy between the microtubule-based transport and the actin-based
transport. Thus, in the dispersion of the pigments in the cell, they are transported
first by microtubule system. They are then transported by the actin filament system.
The physiological interpretation is that the transport by the actin system will achieve
more uniform intracellular distribution of the pigments than microtubule system
alone. The kinetics-based mathematical analysis of this interesting phenomenon has
been presented [23].
In another example, small membrane vesicle is propelled by the push of polymerizing actin filament, but not by myosin motor [24]. This is considered very
similar to the intracellular movements of some bacteria (Sect. 7.17).
7.3 Actin
7.3.1 Actin Monomer Structure
As mentioned in the Sect. 7.2.1, actin takes monomeric or filamentous form. The
actin monomer is comprised of a single polypeptide, the primary sequence of which
has been determined (375 amino acids; [25]). Its molecular weight is ~42 kDa. The
crystal structure of actin monomer that had been complexed with a protein DNase I
to prevent polymerization during the crystallization, has been solved. The monomeric actin has been classified as a globular protein; its dimension is 55 Â 55 Â 35 Å
([26]; Fig. 7.10). The monomer is consisted of two lobes. In the cleft between two
lobes, one nucleotide is bound (ADP when the monomer is incorporated in actin
filament and ATP for the monomer in solution with submillimolar ATP). In vivo
ATP is thought to be the nucleotide bound to monomeric actin, because the
concentration of ATP (Mg-ATP in cytoplasm) exists at a millimolar concentration
and is much higher than ADP. In vitro the bound nucleotide forms a complex with
one Ca
2+ ion, but this Ca
2+ ion is replaced by an Mg
2+ ion when excess Mg
2+ ions
are exogenously added [27]. According to the crystal structure, the nucleotide and
the Ca
2+ ion are linked to the protein matrix through a number of hydrogen and
electrostatic bonds [28]. The two lobes are further divided into subdomains I to
IV. Residues 1-32, 70-144, 338-375 are included in the subdomain I, residues 33-69
in the subdomain II, residues 145-180 and 270-337 in the subdomain III and residues
181-269 in the subdomain IV. This indicates that the polypeptide chain of actin
monomer is folded in a complex manner and partially penetrates into other domains.
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7 Moving Life
cytoplasmic dynein [20] utilize a microtubule as the track; the kinesin and cytoplasmic dynein run in the opposite directions. The run length of the organelle on the
microtubule track is longer than that on the actin track, and the velocity of the
transport on microtubule is generally faster than that on actin filament (as in the
example shown in Fig. 7.9c), or in the case of the transport of color pigments in
melanocyte [21, 22]). The transport of the color pigment is remarkable, because
there is a synergy between the microtubule-based transport and the actin-based
transport. Thus, in the dispersion of the pigments in the cell, they are transported
first by microtubule system. They are then transported by the actin filament system.
The physiological interpretation is that the transport by the actin system will achieve
more uniform intracellular distribution of the pigments than microtubule system
alone. The kinetics-based mathematical analysis of this interesting phenomenon has
been presented [23].
In another example, small membrane vesicle is propelled by the push of polymerizing actin filament, but not by myosin motor [24]. This is considered very
similar to the intracellular movements of some bacteria (Sect. 7.17).
7.3 Actin
7.3.1 Actin Monomer Structure
As mentioned in the Sect. 7.2.1, actin takes monomeric or filamentous form. The
actin monomer is comprised of a single polypeptide, the primary sequence of which
has been determined (375 amino acids; [25]). Its molecular weight is ~42 kDa. The
crystal structure of actin monomer that had been complexed with a protein DNase I
to prevent polymerization during the crystallization, has been solved. The monomeric actin has been classified as a globular protein; its dimension is 55 Â 55 Â 35 Å
([26]; Fig. 7.10). The monomer is consisted of two lobes. In the cleft between two
lobes, one nucleotide is bound (ADP when the monomer is incorporated in actin
filament and ATP for the monomer in solution with submillimolar ATP). In vivo
ATP is thought to be the nucleotide bound to monomeric actin, because the
concentration of ATP (Mg-ATP in cytoplasm) exists at a millimolar concentration
and is much higher than ADP. In vitro the bound nucleotide forms a complex with
one Ca
2+ ion, but this Ca
2+ ion is replaced by an Mg
2+ ion when excess Mg
2+ ions
are exogenously added [27]. According to the crystal structure, the nucleotide and
the Ca
2+ ion are linked to the protein matrix through a number of hydrogen and
electrostatic bonds [28]. The two lobes are further divided into subdomains I to
IV. Residues 1-32, 70-144, 338-375 are included in the subdomain I, residues 33-69
in the subdomain II, residues 145-180 and 270-337 in the subdomain III and residues
181-269 in the subdomain IV. This indicates that the polypeptide chain of actin
monomer is folded in a complex manner and partially penetrates into other domains.
106
7 Moving Life