Where AC proteins (red) bind, the structure of an actin filament (yellow) resembles its depolymerizing end.
That end—the so-called pointed end—normally depolymerizes in the cell even as actin monomers are added at the other “barbed” end. This treadmilling is disturbed by ADF/cofilin (AC) proteins, which tear down actin filaments near the leading edge of migrating cells. The authors used electron microscopy and three-dimensional reconstructions to investigate how AC proteins promote instability and dismantle filaments.
AC-bound filaments looked like the pointed ends of naked actin filaments. Both were missing a contact between subdomain (SD) 2 of one actin monomer and SD1 of the monomer just above it in the filament. In contrast, strong SD1–SD2 interactions were found throughout the rest of the naked actin filaments. These contacts must be weakened for actin strands to adopt a tilted conformation that favors destabilization. AC proteins thus induce this naturally unstable state wherever they bind and cause either depolymerization (if subunits fall off the ends) or severing (if a filament segment is broken off). ▪
