Key result
Actin mutation R256H alters filament conformation, driving instability, delayed nucleation, and formin misregulation.
Why the study?
More than 30 mutations in ACTA2 cause autosomal dominant thoracic aortic aneurysm and dissection, but the molecular effects of the prevalent R256H mutation are not well understood.
The TAAD-causing R256H actin mutation alters filament conformation, resulting in filament instability and misregulation by formin, providing a potential molecular mechanism for the disease.
More than 30 mutations in ACTA2, which encodes α-smooth muscle actin, have been identified to cause autosomal dominant thoracic aortic aneurysm and dissection. The mutation R256H is of particular interest because it also causes patent ductus arteriosus and moyamoya disease. R256H is one of the more prevalent mutations and, based on its molecular location near the strand-strand interface in the actin filament, may affect F-actin stability. To understand the molecular ramifications of the R256H mutation, we generated Saccharomyces cerevisiae yeast cells expressing only R256H yeast actin as a model system. These cells displayed abnormal cytoskeletal morphology and increased sensitivity to latrunculin A. After cable disassembly induced by transient exposure to latrunculin A, mutant cells were delayed in reestablishing the actin cytoskeleton. In vitro, mutant actin exhibited a higher than normal critical concentration and a delayed nucleation. Consequently, we investigated regulation of mutant actin by formin, a potent facilitator of nucleation and a protein needed for normal vascular smooth muscle cell development. Mutant actin polymerization was inhibited by the FH1-FH2 fragment of the yeast formin, Bni1. This fragment strongly capped the filament rather than facilitating polymerization. Interestingly, phalloidin or the presence of wild type actin reversed the strong capping behavior of Bni1. Together, the data suggest that the R256H actin mutation alters filament conformation resulting in filament instability and misregulation by formin. These biochemical effects may contribute to abnormal histology identified in diseased arterial samples from affected patients.
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Malloy et al. (2012) studied Thoracic aortic aneurysm and dissection. R256H actin mutation vs. Wild type actin was evaluated on Actin filament stability and regulation by formin. The R256H actin mutation altered filament conformation, resulting in filament instability, delayed nucleation, and misregulation by formin in a yeast model.
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