Abstract
Post-translational modifications (PTMs) frequently play a key role in regulating protein function. The proteasome is composed of at least 33 different subunits and is a multi-catalytic protease complex that degrades ubiquitinated proteins in eukaryotic cells. The comprehensive PTM analysis of proteasome subunits using proteomic techniques indicated that 21, 1, 1 and 28 subunits were N-acetylated, N-myristoylated, N-methylated and phosphorylated, respectively. Furthermore, the study using the yeast mutants showed that N-acetylation might be involved in the chymotrypsin-like activity and accumulation level of the 20S proteasome and that N-methylation of Rpt1 might be essential for cell growth or stress tolerance in yeast. Like this, it is evident that the PTMs significantly affect the function of the proteasome. The phosphorylation of heterogeneous nuclear ribonucleoprotein K (hnRNP K) is also thought to play an important role in cell regulation and signal transduction. However, the relationship between hnRNP K phosphorylation and cellular events has only been indirectly examined, and the phosphorylated forms of endogenous hnRNP K have not been biochemically characterized in detail. The PTM analysis using 2-D Phos-tag affinity electrophoresis was successful to characterize multiple forms of hnRNP K produced by alternative splicing of the single hnRNP K gene and phosphorylation of Ser116 and/or Ser284. Furthermore, this analysis demonstrated that each form of hnRNP K was differentially modulated in response to external stimulation with bacterial lipopolysaccharide or serum. Like this, the PTM analysis is also crucial for a better understanding of the functional properties of proteins.
