| Comments |
P.295 left column bottom paragraph: "HeLa cells have 10^7 ribosomes and a protein content equivalent to 2 × 10^9 copies of a typical protein of 600 residues (referred to, respectively, simply as ‘cells’ and ‘proteins’ in what follows)[refs 9,10] (Table 1). Since the speed of translation is about five residues per second, protein translation takes about two minutes on average. If all ribosomes are going full tilt all of the time, cells can produce 5 × 10^6 proteins per minute. In the course of each cell cycle (24 h), all cellular proteins must be replaced, in addition to the ∼50% of proteins that are degraded by proteasomes within 24 h of their synthesis [ref 11]. (It is difficult to know to what extent turnover of these proteins is based on stochastic versus qualitative defects. [Investigator] therefore term[s] these proteins ‘retirees’, with a nod to the likelihood that both processes are active.) This translates into a synthetic rate of 5 × 10^9 proteins per 24 h, or 2 × 10^6 proteins synthesized per minute. Accounting for a DRiP [defective ribosomal products] rate of ∼33% in HeLa cells increases this number to 3 × 10^6 proteins synthesized per minute, with the interesting prediction that ribosomes toil with little respite (Fig. 1)." See note beneath figure |