HeLa cell protein synthesis, rate

An alternative type of explanation for the specific discrimination against host cell protein synthesis in poliovirus-infected cells stemmed from the observation that initiation of protein synthesis could be selectively inhibited in HeLa cells and in poliovirus-infected HeLa cells by increasing the osmolarity of the growth medium (Sa-borio et al., 1974). The inhibition was independent of the solute used to increase the osmolarity. However, virus-directed protein synthesis was observed to be relatively more resistant to inhibition by hypertonic medium than was cellular protein synthesis, a fact which was interpreted as indicating that initiation of viral RNA translation was intrinsically more efficient than that of cellular mRNA (Nuss et al., 1975). These workers, therefore, proposed that the virus-specific or virus-induced factor involved in suppression of host protein synthesis could function by indiscriminantly lowering the rate of peptide chain initiation. Under such conditions, translation of viral mRNA, when it was synthesized, could occur due to its inherently strong affinity... 

One way of searching for the presence of inhibitors of polypeptide initiation in infected cells was to add cytoplasmic fractions from virus infected cells to a cell-free system from rabbit reticulocytes. This system initiates the synthesis of new polypeptide chains at a very high rate. Cytoplasm from poliovirus infected HeLa cells, but not from uninfected cells, inhibited protein synthesis in the reticulocyte lysate (59) The inhibitor was isolated and identified as double-stranded (ds) RNA (60). To study the effect of ds RNA on host and viral protein synthesis, a cell-free system from HeLa cells was developed which initiated translation on endogenous cellular or viral mRNA. When added to this system, the ds RNA was found to inhibit the translation of both cellular and viral mRNAs (61). Furthermore, measurement of the amount of ds RNA present in cells early in infection (61, 62) revealed that an insufficient quantity was present to act as a direct agent of protein synthesis inhibition. 

The glucocorticoid induction of HeLa cell alkaline phosphatase requires concomitant protein synthesis [100], but immunoprecipitation and radioimmunoprecipitation studies strongly suggest that the quantities of alkaline phosphatase antigen and its rate of synthesis are not altered by the steroid inducer [101]. Physical and biochemical differences have been observed between the uninduced or basal enzyme and the induced form [101]. Thus, the induction does not seem to be due to an increased rate of de novo phosphatase synthesis but more likely is due to the conversion of an antigenic precursor to a catalytically active (or more active) molecule. The conversion process apparently is inhibited... 

Fig. 1. Rates of protein synthesis in poliovirus-infected and mock-infected HeLa cells. The curves shown were generated from numerous experiments in which HeLa cells were incubated for 15 min with radiolabeled amino acids at various times postinfection or mock infection with poliovirus. The cells were harvested immediately following the labeling period, and incorporation of amino acids into macromolecular protein was determined by scintillation spectroscopy of TCA-precipitable material.

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