Differential N-terminal processing of beta and gamma actin in vivo

Actin is one of the most essential and abundant intracellular proteins, playing an essential physiological role as the major constituent of the actin cytoskeleton. Two cytoplasmic actins, beta- and gamma-actin, are encoded by different genes, but their amino acid sequences differ only by four conservative substitutions at the N-terminus, making it very difficult to dissect their individual regulation in vivo. The majority of actins are N-terminally acetylated, following the removal of N-terminal Met. Here, we analyzed beta and gamma cytoplasmic actin N-termini in vivo and found that beta actin, unlike gamma actin, specifically undergoes sequential removal of N-terminal amino acid Asp residues. This processing affects [~]1-3% of beta actin in different cell types. We identified candidate enzymes capable of mediating this type of processing, and used CRISPR/Cas-9 to delete them, individually or together, in mammalian cell lines. This deletion abolishes most of the beta actin N-terminal processing and results in changes in F-actin levels, cell spreading, filopodia formation, and cell migration, suggesting that the beta actin processing mediated by these enzymes is physiologically important to beta actin function. We propose that selective N-terminal processing of beta actin by sequential removal of Asp contributes to differentiating the functions of non-muscle actin isoforms in vivo.