Functional characterisation of a mutant actin (Met132Val) from a patient with nemaline myopathy.

The mutation Met132Val in the ACTA1 gene was identified in a patient with mild nemaline myopathy (NM). We examined actin mRNA and protein from biopsy samples. Sixty-one percent of the mRNA from the biopsy was not cleaved with BstX1, indicating the presence of mutant messenger in vivo. Monomeric actin was extracted from 2.5 mg of mutant muscle and wild type muscle. A proportion of the NM actin did not polymerise in 50 mM KCl, 2.5 mM MgCl2 but all the wild-type actin did. NM actin was fully polymerised by 50 mM KCl, 2.5 mM MgCl2, 150 nM rhodamine-phalloidin. Thin filaments reconstituted with this co-polymer were different from wild-type. The NM actin produces faster sliding of thin filaments at pCa5 and higher relative isometric force. We conclude that the mutant mRNA and protein is expressed and that the mutation reduces polymerisability and alters thin filament function.