Two forms of a common protein called actin behave differently in cells, despite having almost identical amino-acid sequences. Anna Kashina at the University of Pennsylvania in Philadelphia and her colleagues have uncovered the reason why: one is synthesized, or translated, by the cell from RNA more slowly than the other.

β-actin often has an extra copy of the amino-acid arginine added after translation. However, γ-actin is not found in an arginylated form. By monitoring the metabolic fates of the two forms in various cellular tests, the team showed that γ-actin is translated, and therefore folded, more slowly. The delay reveals a normally hidden residue of another amino acid, lysine. If γ-actin is arginylated, the combination of this and the exposure of the lysine ultimately marks γ-actin for degradation. The two forms of actin are encoded by different genes, which explains the difference in translation rate.

Science 329, 1534–1537 (2010)