Subcellular Distribution of Aminoacyl‐tRNA Synthetases in Various Eukaryotic Cells

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Abstract
The total amount, size distribution and binding of aminoacyl‐tRNA synthetases to ribosomes in a variety of mammalian and avian cells was studied under standard conditions of sample preparation and assay. Aminoacyl‐tRNA synthetases appear to exist in three general forms; ‘free’ enzyme of about 4–9 S, one or more ‘enzyme complexes’ of about 18–25 S, and in association with ribosomes. The aminoacyl‐tRNA synthetase activity for many individual amino acids was surprisingly similar in cell types chosen to be diverse with respect to differentiation state, transformation, and growth rate. Total activity for all amino acids varied about 4‐fold, based on a constant volume of cells. Embryonic tissues had a comparatively high proportion of total synthetase activity associated with ribosomes, whereas this value was relatively low for mouse liver. Distinctive distribution patterns with common and variable features were observed for individual enzymes. The only aminoacyl‐tRNA synthetases found not to be associated in significant amounts with either 18–25‐S enzyme complexes or ribosomes in any of the cell types examined were the enzymes for alanine, histidine, and serine. All cell types evidenced 18–25‐S synthetase activity for arginine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine, methionine, proline, and valine, although in quite variable proportions of the total activity observed for these amino acids. For example, of the valyl‐tRNA synthetase activity not associated with ribosomes, 35% and 100% were found to sediment at 18–25 S in Friend leukemia cells and mouse liver respectively. All cells had two easily distinguishable peaks of arginyl‐tRNA synthetase activity at 4–9 S and 18–25 S respectively; however, the relative proportion of enzyme activity in the peaks differed between cell types. Phenylalanyl‐tRNA synthetase was not observed to occur in an 18–25‐S complex in any of the cell types examined but was bound to ribosomes in variable but generally relatively high proportions. Numerous other specific differences are described. No underlying physiological or biochemical principle has been recognized to account for the specific distribution patterns observed. However, they may reflect variations in cellular architecture that may be related to regulation of protein synthesis.