Global assessment of protein turnover in recombinant antibody producing myeloma cells

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 13C6, 15N4-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 2 h. 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.