Fig. 1. Alpha cardiac actin is arginylated in vivo. (A) Areas of a Coomassie-stained 2D gel of protein samples obtained by fractionation of whole E12.5 mouse hearts from wild-type (+/+, top) and Ate1 knockout (-/-, bottom) embryos under a shallow pH gradient (pH 4-8) to enable separation of individual actin isoforms. pH increases from left to right. Arrowheads indicate the position of spots that were used for the horizontal alignment of the two gels to enable observation of gel shifts of individual actin spots. Arrows indicate the position of the 43 kDa marker, equivalent to the molecular weight of intact alpha actin. , the position of alpha cardiac actin as identified by mass spectrometry. (B) Three-dimensional structure of an alpha cardiac actin monomer (PDB identifier 1J6Z) with post-translationally arginylated sites (R) indicated in pink within the blue actin backbone. The site indicated in pale pink, for which the mass spectrum is not shown, will be described elsewhere.