Ribosomal protein uL15 (previously known as L15 in bacteria, L28 in yeasts and L27A in other eukaryotes) has one of the longest non-globular extension among all ribosomal proteins. The globular domain of uL15 is located in the vicinity of the central protuberance, on the cytosol-exposed side of the large ribosomal subunit, although it is unknown whether uL15 participates in ribosome association with translation factors.
uL15 is a clear example of a protein that has a comparable size but dissimilar structure in different species. Thus, although sequences alignments suggest that its N-terminal extension is conserved, comparison of crystal structures illustrates that this segment ... a similar size of a protein in different species ... with the most divergent structure across major taxonomic groups: thus, whereas globular domain of uL15 is structurally conserved across species, the N-terminal extension of uL15 has dissimilar and unrelated structures in bacteria and eukaryotes. Previous studies indicate that this structural divergence reflects the fact that in eukaryotic species the N-terminal extension aquired an additional function: it serves as a nuclear localization signal to facilitate uL15 delivery to cell nucleoli – the ribosome biogenesis site.