pre40S

pre40S refers to immature particles of the small ribosomal subunit (40S) formed during ribosome biogenesis in eukaryotic cells. These pre40S particles are assembly intermediates that typically harbor the 20S pre-rRNA together with ribosomal proteins and a set of assembly factors. They are produced in the nucleolus and nucleus and are exported to the cytoplasm for final maturation into functional 40S subunits.

During ribosome biogenesis, transcription of ribosomal RNA yields precursors that are processed and assembled into pre40S particles. The 20S pre-rRNA within these particles is subsequently processed to mature 18S rRNA in a cytoplasmic maturation step, aided by various ribosome assembly factors. The pre40S particle is exported from the nucleus to the cytoplasm by export receptors such as CRM1/XPO1, where final maturation steps occur to produce a functional 40S subunit. The maturation process involves structural rearrangements and endonucleolytic/phosphodiester cleavages that finalize the 18S rRNA and the associated ribosomal proteins.

The proper formation and maturation of pre40S particles are essential for the accuracy of translation and overall cellular viability. Disruptions to pre40S biogenesis can lead to reduced protein synthesis and, in humans, are linked to ribosome-related disorders and altered disease risk. Because of their dynamic nature, pre40S particles are a focus of structural, genetic, and biochemical studies aimed at understanding the assembly and quality control of ribosomes.