In situ structure of a dimeric hibernating ribosome from a eukaryotic intracellular pathogen

Abstract

Translational control is an essential process for the cell to adapt to varying physiological or environmental conditions. To survive adverse conditions such as low nutrient levels, translation can be shut down almost entirely by inhibiting ribosomal function. Here we investigated eukaryotic hibernating ribosomes from the microsporidian parasite Spraguea lophii in situ by a combination of cryo-electron tomography (cryoET) and single particle cryoEM. We show that microsporidian spores contain ribosomes primed for host cell invasion and thus shed new light on the infection mechanism of this important pathogen. Prior to host infection, virtually all ribosomes are locked in the 100 S dimeric state, which appears to be formed by a unique dimerization mechanism that is distinct from its bacterial counterparts. Within the dimer, the hibernation factor MDF1 is bound within the E site, locking the L1 stalk in a closed conformation, and thus preventing the translation of mRNAs to polypeptides.

Ribosome clustering and surface layer reorganization in the microsporidian host-invasion apparatus

“…In contrast, the other adjacent ribosomes (d2, d3) are approximately 300 Å distant, on average. An inter-ribosome distance of 220 Å matches well with the observed 100S hibernating ribosome dimer from Spraguea lophii (McLaren et al, 2022), suggesting that the parallel spiral is composed of repetitive sets of hibernating ribosomes organized into an almost crystalline-like arrangement inside the tube.…”

“…Collectively, akin to hibernating ribosome dimers in bacteria (Franken et al, 2017;Matzov et al, 2019), the spiral of ribosome dimers in microsporidia may aid in ribosome hibernation, further adding to the atypical translation regulation mechanisms seen in microsporidian ribosomes (Barandun et al, 2019;Ehrenbolger et al, 2020). Alternatively, these spirals could represent a transient state of polyribosomes seen in pre-spore stages of intracellularly growing microsporidia (Bacela-Spychalska et al, 2018) and precursors to the free hibernating ribosome dimers (McLaren et al, 2022). This would also provide an efficient strategy for ribosome packing, transportation, and redeployment, wherein rapidly translocating arrayed ribosomes may induce ribosome crowding (Vande Broek et al, 1999;Karpova & Gillet, 2018;Maheshwari et al, 2023) for a translation jumpstart to maximize protein synthesis rates.…”