narrowed upon polymerization, which will decrease the rate of exchange of the
bound nucleotide and the divalent cation. According to the reconstruction [31],
subdomain II has been found to move most (~6 Å) upon incorporation of the
monomer into the middle of the filament, and other subdomains move to a lesser
extent (1–2 Å).
The presence of the structural polarity was first demonstrated by the binding of
heavy meromyosin, a proteolytic fragment derived from myosin, to actin filament in
the absence of ATP. The actin filament bound by heavy meromyosin possessed a
characteristic appearance called arrowhead structure, according to which the pointed
and barbed filament ends were distinguished and were named (Fig. 7.12; [32]). A
recent electron microscopic study has shown that the tertiary structure of the terminal
monomer of the barbed end is slightly distorted, which may be the basis of the
distinction between the two ends [33].
Barbed
end
Pointed
end
II
IV
I
III
2.75 nm
5.5 nm
72 nm
b
a
Fig. 7.11 A highly schematic representation of the structure of an actin filament deduced from
electron microscopy and the arrangement of individual monomers in the filament. Panel a, the
electron micrograph has suggested that the actin protomers in the filament is arranged as a two-start
helix (half helix is indicated with dashed line). However, the protomers actually aligned along a
one-start helix (the centers of the protomers are traced with a zig-zag dotted line); the ~166
turn of
a monomer relative to the previous monomer around the filament axis is the reason for the longpitch helix. The rise of the one-start helix is 2.75 nm; the inter-protomer distance along the filament
axis is thus ~5.5 nm. In one pitch of the two-start helix (~72 nm) thirteen protomers are included.
Panel b, a highly schematic representation of the arrangement of the subdomains in the filament
structure; for simplicity, the helical feature is omitted and only the two lobes, each consisted of the
subdomain II and III or the subdomain I and IV, are drawn. The barbed and pointed ends can be
distinguished by the polarity of bound heavy meromyosin (see text and Fig. 7.12) and are labeled in
this figure
108
7 Moving Life
bound nucleotide and the divalent cation. According to the reconstruction [31],
subdomain II has been found to move most (~6 Å) upon incorporation of the
monomer into the middle of the filament, and other subdomains move to a lesser
extent (1–2 Å).
The presence of the structural polarity was first demonstrated by the binding of
heavy meromyosin, a proteolytic fragment derived from myosin, to actin filament in
the absence of ATP. The actin filament bound by heavy meromyosin possessed a
characteristic appearance called arrowhead structure, according to which the pointed
and barbed filament ends were distinguished and were named (Fig. 7.12; [32]). A
recent electron microscopic study has shown that the tertiary structure of the terminal
monomer of the barbed end is slightly distorted, which may be the basis of the
distinction between the two ends [33].
Barbed
end
Pointed
end
II
IV
I
III
2.75 nm
5.5 nm
72 nm
b
a
Fig. 7.11 A highly schematic representation of the structure of an actin filament deduced from
electron microscopy and the arrangement of individual monomers in the filament. Panel a, the
electron micrograph has suggested that the actin protomers in the filament is arranged as a two-start
helix (half helix is indicated with dashed line). However, the protomers actually aligned along a
one-start helix (the centers of the protomers are traced with a zig-zag dotted line); the ~166
turn of
a monomer relative to the previous monomer around the filament axis is the reason for the longpitch helix. The rise of the one-start helix is 2.75 nm; the inter-protomer distance along the filament
axis is thus ~5.5 nm. In one pitch of the two-start helix (~72 nm) thirteen protomers are included.
Panel b, a highly schematic representation of the arrangement of the subdomains in the filament
structure; for simplicity, the helical feature is omitted and only the two lobes, each consisted of the
subdomain II and III or the subdomain I and IV, are drawn. The barbed and pointed ends can be
distinguished by the polarity of bound heavy meromyosin (see text and Fig. 7.12) and are labeled in
this figure
108
7 Moving Life