The global turnover rates of cellular proteins and the secretion rate of a recombinant immunoglobulin G (IgG) in a myeloma cell line, NS0, were determined using SILAC proteomic analysis. After complete labeling of cellular proteins with 13C 6, 15N 4-arginine, cells were transferred to unlabeled medium and the decay of the labeled arginine in proteins was monitored during exponential cell growth. After PAGE separation and mass-spectrometric identification of proteins, those detected with high confidence over at least three time points were used for the determination of turnover rates. Among the 224 proteins quantified with a protein half-life, about 15% have a degradation rate constant lower than one-tenth of specific growth rate. For most proteins, the turnover rate is insignificant in its overall dynamics. Only 6.3% of proteins have a half-life shorter than the cell doubling time. For IgG secretion, both heavy and light chain molecules follow the same kinetic behavior with a half-life estimated to be 2h. The label decay curve appears to show a second region with very slow kinetics, raising the possibility of two populations of IgG molecules with different secretion characteristics.
Bibliographical noteFunding Information:
We thank Dr. Lorraine B. Anderson, Dr. LeeAnn Higgins and Dr Bruce Witthuhn of the Center for Mass Spectrometry and Proteomics (University of Minnesota, USA) for their assistance with mass spectrometry. Bioinformatics support was provided by the University of Minnesota Supercomputing Institute.
- Protein turnover
- Recombinant IgG