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When the tRNA of mammalian cells is incompletely charged due to amino acid deficiency or by analogs which cannot be activated, many metabolic events become limited. This rapid demise of cell function appears to be because of the inhibition of phosphofructokinase (PFK) by uncharged tRNA (FEBS Lett. 302: 113 (1992)). Charged tRNA has been shown to be "sequestered within the protein synthetic machinery", (Negrutskii, B.S. and Deutscher, M.P., Proc. Natl. Acad. Sci. USA 89 3601 (1992) and would therefore be removed from an inhibitory role. Besides the direct demonstration that tRNA inhibits PFK in an assay regarded as indicative of its control mechanism, several reports in the literature support this model. These include 1) The rapid onset of inhibition of glycolysis and glucose uptake by intact cells upon amino acid deficiency and the similar lesion at the 43S ribosomal subunit on glucose or amino acid deprivation. 2) The recognition that unusually high concentrations of cAMP required to stimulate protein synthesis in energy depleted or gel filtered lysates correlates with its action on PFK as an analog of the positive effector, adenosine-5'-monophosphate. 3)The often repeated observation that the product of PFK activity, fructose-1,6-diphosphate, is a stimulant of protein synthesis (see Jackson, R.J., et al. Eur. J. Biochem. 131: 289-313 (1983)). This diphosphate has been shown to be the proximate effector binding to eIF-2B, the guanine nucleotide exchange factor (Singh, L.P. Arror, A.R. and Wahba, A.J., FASEB J. 8 279 (1994)) which by releasing GDP bound to the inactive GDP:eIF-2 complex, permits the factor to initiate a new peptide chain. The above information supports the view that the block at the G1 restriction point in the cell cycle of normal cells brought about by amino acid deprivation is a result of inhibition of protein synthesis through the phosphofructokinase-uncharged tRNA mechanism. This is consistent with observations in the literature that tumor and transformed cells, which are more resistant to this block (Pardee, A.B., Proc. Natl. Acad. Sci. U.S.A. 71:1286-1291 (1974)) have a higher phosphofructokinase activity or higher levels of fructose-1,6-diphosphate. |
