Tmod overexpression (red, bottom) arrests filament extension before it reaches the M line.

When it comes to actin dynamics, the pointed end of the actin filament is the forgotten cousin. The barbed end is in a state of constant flux in motile cells, pushing out the front of the cell, whereas the pointed end rarely rates a mention. But now, Mardahl-Dumesnil and Fowler report that in the actin-containing thin filaments of developing fly muscles, which grow at a more leisurely pace, the action is at the pointed end (page 1043).

The authors target the pointed end by transiently overexpressing the fly version of Tropomodulin (Tmod) in indirect flight muscles (IFM). If the Tmod is overexpressed during pupation when myofibrils are assembled, a central core of thin filaments is shorter than normal, which in the most serious cases results in adults that are unable to fly. The short filaments are permanently arrested—they do not resume their growth once Tmod levels decline to near normal—and a fraction of the overexpressed form of Tmod stays as a permanent cap on the pointed end.

Fowler believes that the excess Tmod overwhelms a factor that normally makes Tmod capping of pointed ends dynamic. By the time the Tmod levels decline again, this factor may be far away in the central M line, conveyed from the foreshortened thin filaments by the continued elongation of the thick filaments of myosin. The outer thin filaments are not affected by the overexpression only because they are born later, after Tmod levels have returned to normal.▪