A Subcellular Map of Translational Machinery

Revealing how ribosomes are organized, specialized, and regulated inside cells

For decades, ribosomes—the molecular machines that synthesize proteins—were viewed as uniform and passive participants in gene expression. This view assumed that all ribosomes were functionally equivalent, translating mRNAs without selectivity or spatial regulation. However, emerging evidence has suggested that ribosomes can vary in composition and may contribute actively to gene regulation. What has been missing is the ability to directly visualize and define ribosomes within the complex architecture of living cells.

In this study, we overcome this fundamental limitation by developing two complementary technologies—RiboExM (ribosome expansion microscopy) and ALIBi (optogenetic proximity labeling)—that together enable the visualization and molecular characterization of ribosomes at single-molecule resolution within intact cells. These tools allow us to move beyond bulk measurements and instead build a spatially resolved, molecular map of translation across subcellular compartments.

Using RiboExM, we generated the first panoramic, single-molecule map of ribosomes inside mammalian cells. By physically expanding cells and imaging ribosomal subunits with nanoscale precision (~25 nm resolution), we could distinguish individual ribosomal components, assembled ribosomes, and actively translating polysomes. This revealed a striking and previously unappreciated spatial organization of the translational machinery. For example, small (40S) ribosomal subunits are broadly distributed throughout the cytoplasm, whereas large (60S) subunits cluster near sites of active translation and are strongly enriched at the endoplasmic reticulum (ER) .

These observations suggest that translation is not uniformly distributed but instead occurs within discrete subcellular “hotspots”, where ribosomes and associated factors are locally concentrated. Indeed, polysomes—clusters of actively translating ribosomes—form spatially defined regions that dynamically respond to cellular conditions. This organization implies that cells regulate protein synthesis not only through mRNA abundance but also through spatial control of ribosome availability and assembly.

To complement imaging, we developed ALIBi, an optogenetic method that enables precise biochemical isolation of ribosomes from specific subcellular locations. By combining light-controlled biotin labeling with proteomics and RNA sequencing, ALIBi allows us to define the molecular composition of ribosomes, including their associated proteins (RAPs) and bound mRNAs, in distinct cellular compartments.

Using these tools together, we uncovered multiple layers of ribosome specialization. At the ER, we identified a previously unrecognized enrichment of 60S subunits and specific ribosome-associated factors, including the late-stage ribosome biogenesis protein Lsg1. Functional experiments revealed that Lsg1 plays a critical role in tethering 60S subunits to the ER and regulating the translation of specific classes of mRNAs—particularly those encoding membrane and secreted proteins—through a mechanism independent of classical ER stress pathways . This demonstrates that ribosome localization itself can directly influence which proteins are synthesized.

Beyond the ER, we discovered that ribosomes associated with the outer mitochondrial membrane (OMM) exhibit distinct compositions, lacking specific ribosomal proteins and preferentially translating mRNAs involved in metabolic pathways. This provides direct evidence that ribosome heterogeneity contributes to metabolic regulation at organelle interfaces, linking ribosome composition to functional specialization.

We also extended these analyses to neurons, where local translation is essential for synaptic function. RiboExM revealed a dynamic balance between monosomes and polysomes in neuronal processes: monosomes predominate at baseline, while polysomes rapidly assemble in response to stimulation. This finding highlights a mechanism by which neurons regulate protein synthesis in space and time, enabling rapid, localized responses to external signals.

Together, these results establish that ribosomes are not uniform machines but instead form diverse, spatially organized populations with distinct compositions and functions. Translation emerges as a highly compartmentalized and regulated process, shaped by the localization, composition, and interactions of ribosomes within the cell.

More broadly, this work provides a conceptual and technological framework for understanding gene expression beyond the genome and transcriptome. By revealing how ribosome heterogeneity and spatial organization contribute to selective translation, our findings redefine the ribosome as an active regulator of cellular identity and function. These insights have far-reaching implications for development, neurobiology, metabolism, and disease, where dysregulation of translation is increasingly recognized as a central driver of pathology.

Finally, the tools introduced here—RiboExM and ALIBi—offer a generalizable platform for studying macromolecular machines in their native cellular context. They open new avenues for dissecting how molecular composition and spatial organization govern biological function, providing a foundation for future discoveries in both basic and translational biology.

Zhang*, Xu*, et al., Science 2025

Zhang Z, Xu A, Bai Y, Chen Y, Cates K, Kerr C, Bermudez A, Susanto TT, Wysong K, García Marqués FJ, Nolan GP, Pitteri S, Barna M. A subcellular map of translational machinery composition and regulation at the single-molecule level. Science. 2025 Mar 7;387(6738):eadn2623. doi: 10.1126/science.adn2623. Epub 2025 Mar 7. PMID: 40048539.