Purification and separation of the 20S immunoproteasome from the constitutive proteasome and identification of the subunits by LC-MS.

The proteasome is a multicatalytic protease complex present in all eukaryotic cells, which plays a critical role in regulating essential cellular processes. During the immune response to pathogens, stimulation by γ interferon induces the production of a special form of proteasome, the immunoproteasome. Inappropriate increase of proteosomal activity has been linked to inflammatory and autoimmune diseases. Selective inhibition of the immunoproteasome specific LMP7 subunit was shown to block inflammatory cytokine secretion in human PBMC, thus making the immunoproteasome an interesting target to fight autoimmune diseases. This paper describes a method for purification and separation of the 20S immunoproteasomes from the constitutive proteasome, which is ubiquitously present in all cells, based on hydrophobic interaction chromatography. The purified immunoproteasome showed several bands, between 20-30 kDa, when subjected to polyacrylamide gel electrophoresis under denaturing conditions. The purified proteasome complexes had a molecular mass of approximately 700 kDa as estimated by gel filtration. Identification of the catalytic subunits in the immunoproteasomes was performed in Western blot with antibodies directed specifically against either the constitutive or the immunoproteasome subunits. The purified immunoproteasome possessed all three protease activities associated with the proteasome complex. LC/MS analysis confirmed the presence of the three immunoproteasome catalytic subunits in the purified immunoproteasome.

Differences in the glycosylation of recombinant proteins expressed in HEK and CHO cells.

Glycosylation is one of the most common posttranslational modifications of proteins. It has important roles for protein structure, stability and functions. In vivo the glycostructures influence pharmacokinetics and immunogenecity. It is well known that significant differences in glycosylation and glycostructures exist between recombinant proteins expressed in mammalian, yeast and insect cells. However, differences in protein glycosylation between different mammalian cell lines are much less well known. In order to examine differences in glycosylation in mammalian cells we have expressed 12 proteins in the two commonly used cell lines HEK and CHO. The cells were transiently transfected, and the expressed proteins were purified. To identify differences in glycosylation the proteins were analyzed on SDS-PAGE, isoelectric focusing (IEF), mass spectrometry and released glycans on capillary gel electrophoresis (CGE-LIF). For all proteins significant differences in the glycosylation were detected. The proteins migrated differently on SDS-PAGE, had different isoform patterns on IEF, showed different mass peak distributions on mass spectrometry and showed differences in the glycostructures detected in CGE. In order to verify that differences detected were attributed to glycosylation the proteins were treated with deglycosylating enzymes. Although, culture conditions induced minor changes in the glycosylation the major differences were between the two cell lines.