Structural Basis for Capping Protein Sequestration by Myotrophin (V-1)

Capping protein (CP) is a ubiquitously expressed, heterodimeric 62-kDa protein that binds the barbed end of the actin filament with high affinity to block further filament elongation. Myotrophin (V-1) is a 13-kDa ankyrin repeat-containing protein that binds CP tightly, sequestering it in a totally inactive complex in vitro. Here, we elucidate the molecular interaction between CP and V-1 by NMR. Specifically, chemical shift mapping and intermolecular paramagnetic relaxation enhancement experiments reveal that the ankyrin loops of V-1, which are essential for V-1/CP interaction, bind the basic patch near the joint of the α tentacle of CP shown previously to drive most of the association of CP with and affinity for the barbed end. Consistently, site-directed mutagenesis of CP shows that V-1 and the strong electrostatic binding site for CP on the barbed end compete for this basic patch on CP. These results can explain how V-1 inactivates barbed end capping by CP and why V-1 is incapable of uncapping CP-capped actin filaments, the two signature biochemical activities of V-1.