Investigation of Intact Subunit Polypeptide Composition of the 20S Proteasome Complex from Rat Liver Using Mass Spectrometry

Abstract

Proteasomes are multicatalytic proteolytic complexes found in almost all living cells that are responsible for protein degradation in both the cytosol and nucleus. They are involved in many important biological processes, including the removal of abnormal, misfolded or improperly assembled proteins, stress response, cell differentiation, metabolic adaption, and cellular immune response [1, 2]. The 20S proteasome is the catalytic core of the larger, ATP-dependent 26S complex that is responsible for degradation of ubiquitin-conjugated proteins. With a molecular weight of approximately 750 kDa, the 20S proteasome complex has a cylindrical structure consisting of four stacked rings, each of which is organized from seven α and β subunits, assembled in the order αββα(1–4). This overall structure is conserved from archebacteria to eukaryotes, which has been shown clearly in the crystal structures ofthe 20S proteasomes from Thermoplasma acidophilum and Saccharomyces cerevisiae [5,6]. The proteasome complex from archeabacterium Thermoplasma acidophilum contains only two different but related subunits, α and β, while it is known that the eukaryotic proteasome complex is composed of at least 14 subunits with molecular masses of 21 to 34 kDa, and different charges (pI 3–10). These subunits can be divided into α- and β-type based on their polypeptide sequence homology with the T. acidophilum α- or β-subunit. The primary structure of these subunits shows high inter-subunit homology within species, and high evolutionary conservation in various eukaroytes, suggesting that they constitute a multi-gene family and may have originated from a common ancestral gene [4].