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Protein Expression Under Pressure

Cellular protein concentrations are generally under stronger evolutionary pressure than mRNA concentrations. [Also see Report by Khan et al.] Which is more conserved across species—the concentrations of proteins or the concentrations of the messenger RNAs (mRNAs) that encode them? When examining orthologous genes, it's protein concentrations that are more similar to each other. This observation was first made in worm and fly (1), and later for eight organisms ranging from bacteria to yeast, plant, and human (2). However, because the measurement platforms, data sets, and cell samples were heterogeneous in these studies, it has been difficult to separate possible biological trends from technical artifacts. On page 1100 of this issue, Khan et al. (3) show that the biological trend is very real. The authors show that protein concentrations from identical cell types across three primate species are under stronger evolutionary constraints than the respective mRNA expression levels.