Figure 1.

Molecular model of an actin monomer highlighting the position of mutated residues analyzed in this study (the molecular model of alpha actin was based on a 2.8 Å atomic structure of rabbit skeletal muscle actin (RCSB Protein Data Bank 1ATN,³⁶). The ribbon structure showing side chains affected by ACTA1 assessed in this study was created with Swiss-PDB Viewer v4.1.0³⁷). Human alpha skeletal actin is a globular molecule consisting of 2 domains (4 subdomains) which are connected by a “hinge” region (ribbon structure shown in light blue). A cleft is located between the two domains containing a divalent cation and ATP/ADP (yellow). Actin polymerizes into two twisted strands where each actin molecule interacts with four neighboring monomers. The DNase1 binding loop (pale yellow) is involved in actin-actin binding by forming a lock-in-key interaction between neighboring actin molecules. Details on further residues involved in actin-actin interactions can be found in⁵³. Actin binds to a number of proteins which are crucial for its function. Important binding partners include nebulin (amino acids involved in binding are 98–103, 127–132, 361–368, 228–238, 4–8, 350–358, shown in light pink⁵³), tropomyosin (Asp27, Arg30, Arg149, Lys328, Lys329, Lys330, Pro335 and Glu336,⁵⁶, shown in orange) and myosin (via amino acid 2–6, 26–27, 101–102, 146, 334, 335, 343, 351, 354⁵³). The 14 actin residues affected by the mutations we investigated in this study are highlighted in red and blue depending on their effect on the Ca²⁺ sensitivity of force generation (red = reduced Ca²⁺ sensitivity; dark blue = Ca²⁺ sensitivity comparable to controls). Three of four residues reducing Ca²⁺ sensitivity are located within subdomain 4.