The arginylation of -actin regulates the actin cytoskeleton and cell motility, report Kashina and colleagues in Science. The post-translational addition of an Arg residue to the N terminus of a protein is important for embryogenesis, cardiovascular development and angiogenesis. However, until now, the molecular effects of arginylation, which is catalysed by Arg–tRNA protein transferase-1 (ATE1), and the proteins that are arginylated in vivo were largely unknown.

...arginylation can have wide-ranging effects on the molecular and cellular levels through a single protein target.

Actin undergoes N-terminal processing in vivo in such a way that it could become a target for arginylation. By analysing the actin isoforms in whole-cell lysates, the authors showed that Arg can be added to residue Asp3 of -actin. They also estimated that 40% of -actin is arginylated in vivo.

So what effect does this modification have? Arginylation has been proposed to mark proteins for degradation, but the authors found that -actin stability was unaffected by this modification. Furthermore, by comparing immunoprecipitations from wild-type and ATE1^-/- cells, they found that the interactions of -actin with other proteins were unaffected by arginylation. However, they showed that arginylation affected the capability of actin to polymerize; in ATE1^-/--cell extracts, actin filaments clustered and formed filamentous aggregates.

Next, the authors analysed how arginylation affects the intracellular functions of -actin by comparing the morphology, motility and actin cytoskeleton of wild-type and ATE1^-/- cells. In the absence of arginylation, the intracellular -actin distribution was altered and lamella formation was disrupted, which reduced the motility of ATE1^-/- cells. These lamella defects could be rescued by transiently transfecting ATE1^-/- cells with a 'permanently arginylated' -actin construct.

Kashina and co-workers have therefore shown that arginylation can have wide-ranging effects on the molecular and cellular levels through a single protein target. They propose that the arginylation of -actin adds bulky positive charges to actin filaments to prevent them from aggregating. This would facilitate the assembly of a loose actin network at the leading edge and regulate lamella formation in motile cells.

Author: Rachel Smallridge - Copyright © 2006 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission