Detection and Characterization of the Eukaryotic Vacant Ribosome

Upon transcription, most mRNAs associate with the small ribosomal subunit, after which a fully translating ribosome assembles. Under starvation or stress, however, mRNA-ribosome associations are blocked and many mRNAs are instead sequestered with specific RNA-binding proteins into stress granules or other subcellular condensates, a process that has been extensively studied. In contrast, much less attention has been paid to the fate of ribosomes under these same conditions. Ribosomes can remain fully assembled but unbound to mRNA, entering an inactive, dormant state. Dormancy is often supported by specific protein factors which protect ribosomes from degradation and facilitate reactivation once growth conditions improve. In this review, we highlight that dormant ribosome states are well defined in prokaryotes, in part because they possess distinct and experimentally tractable features, such as stable vacant 100S dimers. In eukaryotes, by contrast, analogous disomes are largely absent, making their discovery more indirect and method-dependent. We therefore focus on how evidence for eukaryotic dormant ribosomes has been assembled through multiple independent findings and how their interpretation depends critically on the experimental approaches used to study them. Finally, we consider atypical ribosomal states, such as translationally inactive polysomes in neurons, which underscore the context-dependent nature of ribosome activity.