Tropomodulin Contains Two Actin Filament Pointed End-capping Domains

Tropomodulin 1 (Tmod1) is a ∼40-kDa tropomyosin binding and actin filament pointed end-capping protein that regulates pointed end dynamics and controls thin filament length in striated muscle. In vitro, the capping affinity of Tmod1 for tropomyosin-actin filaments (Kd ∼ 50 pm) is several thousand-fold greater than for capping of pure actin filaments (Kd ∼ 0.1 μm). The tropomyosin-binding region of Tmod1 has been localized to the amino-terminal portion between residues 1 and 130, but the location of the actin-capping domain is not known. We have now identified two distinct actin-capping regions on Tmod1 by testing a series of recombinant Tmod1 fragments for their ability to inhibit actin elongation from gelsolin-actin seeds using pyrene-actin polymerization assays. The carboxyl-terminal portion of Tmod1 (residues 160–359) contains the principal actin-capping activity (Kd ∼ 0.4 μm), requiring residues between 323 and 359 for full activity, whereas the amino-terminal portion of Tmod1 (residues 1–130) contains a second, weaker actin-capping activity (Kd ∼ 1.8 μm). Interestingly, 160–359 but not 1–130 enhances spontaneous actin nucleation, suggesting that the carboxyl-terminal domain may bind to two actin subunits across the actin helix at the pointed end, whereas the amino-terminal domain may bind to only one actin subunit. On the other hand, the actin-capping activity of the amino-terminal but not the carboxyl-terminal portion of Tmod1 is enhanced several thousand-fold in the presence of skeletal muscle tropomyosin. We conclude that the carboxyl-terminal capping domain of Tmod1 contains a TM-independent actin pointed end-capping activity, whereas the amino-terminal domain contains a TM-regulated pointed end actin-capping activity.