Actin Filaments Grow Faster at End Than at

We saw earlier that myosin decoration experiments reveal an inherent structural polarity of F-actin (see Figure 19-4). This polarity is also manifested by the different rates at which ATP-G-actin adds to the two ends. One end of the filament, the (+) end, elongates 5-10 times as fast as does the opposite, or (—), end. The unequal growth rates can be demonstrated by a simple experiment in which myosin-decorated actin filaments nucleate the polymerization of G-actin. Electron microscopy of the elongated filaments reveals bare sections at both ends, corresponding to the added undecorated G-actin. The newly polymerized (undecorated) actin is 5-10 times as long at the (+) end as at the (—) end of the filaments (Figure 19-8a).

reaches Cc.

▲ EXPERIMENTAL FIGURE 19-8 Myosin decoration and capping proteins demonstrate unequal growth rates at the two ends of an actin filament. (a) When short myosin-decorated filaments are the nuclei for actin polymerization, the resulting elongated filaments have a much longer undecorated (+) end than (—) end. This result indicates that G-actin monomers are added much faster at the (+) end than at the (—) end.

(b) Blocking the (+) or (—) ends of a filament with actin-capping proteins permits growth only at the opposite end. In polymerization assays with capped filaments, the critical concentration (Cc) is determined by the unblocked growing end. Such assays show that the Cc at the (+) end is much lower than the Cc at the (—) end. [Part (a) courtesy of T Pollard.]

▲ FIGURE 19-9 Treadmilling of actin filaments. At G-actin concentrations intermediate between the Cc values for the (—) and (+) ends, actin subunits can flow through the filaments by attaching preferentially to the (+) end and dissociating preferentially from the (—) end of the filament. This treadmilling phenomenon occurs in some moving cells. The oldest subunits in a treadmilling filament lie at the (—) end.

The difference in elongation rates at the opposite ends of an actin filament is caused by a difference in Cc values at the two ends. This difference can be measured by blocking one or the other end with proteins that "cap" the ends of actin filaments. If the (+) end of an actin filament is capped, it can elongate only from its (—) end; conversely, elongation takes place only at the (+) end when the (—) end of a filament is blocked (Figure 19-8b). Polymerization assays of such capped filaments have shown that the Cc is about six times lower for polymerization at the (+ ) end than for addition at the (—) end.

As a result of the difference in the Cc values for the (+) and (— ) ends of a filament, we can make the following predictions: at ATP-G-actin concentrations below C+, there is no filament growth; at G-actin concentrations between C+ and C—, growth is only at the (+) end; and, at G-actin concentrations above C—, there is growth at both ends, although it is faster at the (+) end than at the (—) end. When the steady-state phase is reached at G-actin concentrations intermediate between the Cc values for the (+) and the (—) ends, subunits continue to be added at the (+) end and lost from the (—) end (Figure 19-9). In this situation, the length of the filament remains constant, with the newly added subunits traveling through the filament, as if on a treadmill, until they reach the (—) end, where they dissociate. Turnover of actin filaments at the leading edge of some migrating cells probably occurs by a treadmilling type of mechanism, with sub-units added to filaments near the leading edge of the cell and lost from the other end toward the rear.

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