Proteins that cap the ends of the actin filament are essential regulators of
cytoskeleton dynamics. Whereas several proteins cap the rapidly growing barbed
end, tropomodulin (Tmod) is the only protein known to cap the slowly growing
pointed end. The lack of structural information severely limits our
understanding of Tmod's capping mechanism. We describe crystal structures of
actin complexes with the unstructured amino-terminal and the leucine-rich repeat
carboxy-terminal domains of Tmod. The structures and biochemical analysis of
structure-inspired mutants showed that one Tmod molecule interacts with three
actin subunits at the pointed end, while also contacting two tropomyosin
molecules on each side of the filament. We found that Tmod achieves
high-affinity binding through several discrete low-affinity interactions, which
suggests a mechanism for controlled subunit exchange at the pointed end.
