Inhibition of Translation via Phosphorylation by p21‐Activated Protein Kinase 2 (Pak2)

Abstract

Translation is down‐regulated in response to a variety of stresses. Under moderate stress, p21‐activated protein kinase (Pak2) is activated by Cdc42 and induces cytostasis, while caspase 3 cleaves and activates Pak2 under apoptotic conditions. Pak2 inhibits translation through phosphorylation of initiation factor 4G and the serine/threonine kinase Mnk1. The present study shows that Pak2 phosphorylates three ribosomal proteins, S6 and S10 in the small ribosomal subunit and L34 in the large ribosomal subunit. S10 and L34 are phosphorylated by Cdc42‐activated and caspase‐cleaved Pak2, while S6 is phosphorylated only by caspase‐cleaved Pak2 at one site on serine 235. The same site is phosphorylated during early apoptosis following treatment of 293T cells with H2O2. Addition of active Pak2 to a reticulocyte lysate results in the phosphorylation of S6 and S10, inhibiting protein synthesis. Reconstitution of reticulocyte lysate with ribosomes phosphorylated by caspase‐activated Pak2 in vitro, inhibits translation by 33%, as compared with lysate reconstituted with non‐phosphorylated ribosomes. Active and inactive Pak2 binds to ribosomes and 40S ribosomal subunits, but not to 60S subunits. In summary, Pak2 is shown to directly down‐regulate translation through phosphorylation of ribosomal proteins.