  Transcriptional and post-transcriptional regulation shape
tissue-type-specific proteomes, but their relative contributions remain
contested. Estimates of the factors determining protein levels in human tissues
do not distinguish between (i) the factors determining the variability between
the abundances of different proteins, i.e., mean-level-variability and, (ii)
the factors determining the physiological variability of the same protein
across different tissue types, i.e., across-tissues variability. We sought to
estimate the contribution of transcript levels to these two orthogonal sources
of variability, and found that scaled mRNA levels can account for most of the
mean-level-variability but not necessarily for across-tissues variability. The
reliable quantification of the latter estimate is limited by substantial
measurement noise. However, protein-to-mRNA ratios exhibit substantial
across-tissues variability that is functionally concerted and reproducible
across different datasets, suggesting extensive post-transcriptional
regulation. These results caution against estimating protein fold-changes from
mRNA fold-changes between different cell-types, and highlight the contribution
of post-transcriptional regulation to shaping tissue-type-specific proteomes.
