Posttranslational Modification of the 20S Proteasomal Proteins of the Archaeon Haloferax volcanii

Abstract

ABSTRACT 20S proteasomes are large, multicatalytic proteases that play an important role in intracellular protein degradation. The barrel-like architecture of 20S proteasomes, formed by the stacking of four heptameric protein rings, is highly conserved from archaea to eukaryotes. The outer two rings are composed of α-type subunits, and the inner two rings are composed of β-type subunits. The halophilic archaeon Haloferax volcanii synthesizes two different α-type proteins, α1 and α2, and one β-type protein that assemble into at least two 20S proteasome subtypes. In this study, we demonstrate that all three of these 20S proteasomal proteins (α1, α2, and β) are modified either post- or cotranslationally. Using electrospray ionization quadrupole time-of-flight mass spectrometry, a phosphorylation site of the β subunit was identified at Ser129 of the deduced protein sequence. In addition, α1 and α2 contained N-terminal acetyl groups. These findings represent the first evidence of acetylation and phosphorylation of archaeal proteasomes and are one of the limited examples of post- and/or cotranslational modification of proteins in this unusual group of organisms.