Resumo

Aim: Identify possible functional changes in cell surface proteins during human evolution.

Methods: Genomes of modern humans, Neandertal fossils, chimpanzee and orangutan were aligned and scanned for changes in cell-surface proteins. In a first approach, only changes within signal peptides were examined, where Neandertal shared the ancestral allele with the other primate genomes. Reporter proteins carrying the modern and the ancestral signal peptide, tagged with GFP, were transfected in cultured human cells, and the live cells were stained with fluorescent antibodies. The ratio of cell surface to total reporter protein was examined using confocal microscopy and quantified using flow cytometry. In a second approach, neuronal cell surface proteins were ranked by the number and radicality of amino acid changes on the human lineage since the human-chimpanzee split. The cDNA coding for the protein with the highest number of changes was tagged with GFP and expressed in cultured human cells for functional testing.

Results: 10 changes in signal peptides occurred during the most recent period of human evolution, after the modern human-Neandertal split, and 4 of them are fixed in 50 modern humans. Although there are differences in efficiency between signal peptides derived from distinct human genes, none of the recent evolutionary changes affected signal peptide efficiency. Among the neuronal plasma membrane proteins that have evolved since the human-chimpanzee split, one potential cell adhesion protein shows particularly numerous changes.

Conclusions: Most coding changes in human evolution are likely neutral and have become fixed due to drift. Rational selection of candidates followed by functional testing is the best way to identify the few positively selected changes.

Supported by: Max Planck Institute Presidential Fund