Regulatory dynamics of monosomes and polysomes in cellular adaptation

Cytosolic 80S ribosomes have long been considered a uniform translation apparatus, with the only distinction being that they exist as either monosomes or polysomes. However, emerging evidence has revealed that monosomes and polysomes exhibit distinct translational profiles, contributing to selective protein synthesis and localized messenger RNA (mRNA) translation in specific subcellular compartments in mammals. When mammalian cells encounter environmental or metabolic stress, global gene expression is dynamically reprogrammed, accompanied by a marked shift in the monosome-to-polysome ratio, suggesting context-dependent roles for monosome- and polysome-mediated mRNA translation in cellular adaptation. In parallel, accumulating evidence indicates that distinct types of eukaryotic non-translating monosomes are formed under various biological conditions. Moreover, monosomes participate in the first round of mRNA translation on newly synthesized mRNAs, a process coupled with several early events such as nonsense-mediated mRNA decay and messenger ribonucleoprotein (mRNP) remodeling. In this review, we summarize current advances in understanding the roles of monosomes as crucial regulatory layers that shape the proteome across diverse physiological conditions in mammals.