uS3 is located in the head of the small ribosomal subunit. uS3 directly interacts with mRNA during protein synthesis and forms part of the ribosomal mRNA entry site.
Both in bacteria and eukaryotes, uS3 carries an intrinsically disordered C-terminal extension. In eukaryotes, the extension interacts with protein Rack1. In bacteria, the extension is disordered in both crystal and cryo-EM function, and its role in protein synthesis is unknown.
In eukaryotes, uS3 has a number of extraribosomal functions. Among the best described functions is uS3 role in DNA repair: upon DNA-damage response, uS3 is phosphorylated and accumulated in the nucleus of the cell (in a ribosome-free form) where it serves as DNA endonuclease enzyme. It is currently unclear whether uS3 phosphorylation promotes dissociation of uS3 from the ribosome or it prevents nascent uS3 from incorporation into nascent ribosomes during ribosome biogenesis.