Mapping the essential structures of human ribosomal protein L7 for nuclear entry, ribosome assembly and function

Human large subunit protein L7 carries multiple nuclear localization signals (NLS) in its structure: there are three monobasic partite NLSs at the NH2-region of the first 54 amino acid residues and a bipartite in the middle section at position of 156–167. The C-region of the last 50 amino acid residues displays membrane binding nature, and might involve in forming a nuclear microbody for pre-nucleolar ribosome assembly. The middle section covers 144 amino acid residues which are essential for the structure and function of ribosome. This is evident from findings that truncated L7 without the NH2-region or the C-region, or missing both regions, is capable of reaching nucleolus and incorporating in ribosome, however, only ribosomes bearing truncated L7 without the NH2-region is capable of engaging in polysome formation. Combining with the phylogenic findings from homologous sequence alignment, the NH2-region of L7, besides being as a eukaryotic expansion segment, can be excluded from building a functional eukaryotic ribosome.