Protein uS2 surrounds the mRNA channel of the ribosome from the cytosol-exposed side of the small ribosomal subunit. In bacteria, this protein helps recruit mRNA to the ribosome .
In bacteria, uS2 carries an additional helix-loop-helix extension, also know as a coiled-coil domain, which is likely helps recruiting bacteria-specific protein bS1, which in turn recruits mRNAs to the small ribosomal subunit (1,2). Mutations in this coiled-coil domain confer thermosensitive phenotype, which can be suppressed by ribosomal protein bS1 (3).
In eukaryotes, uS2 (also known as S0) carries an additional C-terminal extension. This extension is disordered in crystal and cryo-EM structures of the eukaryotic ribosome, and its role in protein synthesis remain unknown.