We thank the Electron Microscopy, Light Microscopy, Proteomics and Biophysics Core Facilities of the National Heart, Lung, and Blood institute (NHLBI) for their support, advice, and the use of facilities. We thank the laboratory of Philipp Kukura for the mass photometry technology. We thank Fang Zhang for her help with reagent and actin preparation. R.L. is supported by a postdoctoral fellowship from NHLBI (Lenfant Biomedical Fellowship). This work is also supported by the Intramural Research Program of the NHLBI, NIH (HL006049 to J.R.S.).