P protein

by Séverine Altairac

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There is a famous sequence in Quai des Brumes – a very popular French film shot in the 1930s – in which Jean Gabin, subdued by the sky blue of his interlocutor’s gaze, leans over and breathes into Michèle Morgan’s ear, “T’as d’beaux yeux tu sais” – meaning literally: “You’ve got beautiful eyes you know” … though it means far more. The blue of an eye is both fascinating and mysterious, and we are getting closer to an explanation for it. It is common knowledge that the colour of our eyes is due to the accumulation of a pigment in the iris – melanin – whose synthesis depends on the activity of a protein known as P protein. Despite years of research, scientists had not been able to pin down a modification in P protein, which could explain the azure of a look, until recently when the long sought after mutation was discovered – only not at all where they were expecting it!

P protein is essential to pigmentation. Although its precise function is unknown, it is a key factor in the synthesis of melanin – the pigment which gives our skin, our hair and our eyes their colour. The motto goes: the more the melanin, the darker the eyes. Albinism, on the other hand, which results from the discolouration of our skin, eyes and hair occurs following a serious defect in melanin synthesis. To date, approximately 40 mutations have been identified on the P protein gene, which are all involved in one of the major forms of albinism.

Blue eyes have poor melanin content. Convinced that this might have something to do with P protein, researchers carried out a major study in the hope of discovering what it is that makes the difference between brown eyes and blue eyes. To their surprise, they found only one mutation and, what’s more, not on the P protein gene but on an adjacent gene: HERC2. This particular mutation has the same effect as a switch and slows down protein P synthesis, thus reducing its production. Without it, P protein is produced normally and eyes are brown. And with it, P protein is produced in smaller quantities and eyes turn out to be blue.

What this implies, is that all blue-eyed individuals carry the same mutation which seems to have appeared about ten thousand years ago. Before that, everyone had dark eyes. The first blue look appeared on earth during the Neolithic period when our ancestors left the region of the Black Sea for a better future, and built settlements in Northern Europe. Ever since, blue eyes have cleared millennia, gliding successfully through time. Why are blue eyes still around? Why has evolution kept them? White skin – which is a phenotype so typical to our Northern hemisphere – is explained as an adaptation to poor sunlight, thus favouring the synthesis of vitamin D. But blue eyes? What are they for? A number of researchers suggest that it may simply have something to do with sexual selection. Why not? How many of us have got hopelessly lost in an ocean blue gawp ?

UniProt cross references
P protein, Homo sapiens (Human): Q04671
Protein Spotlight (ISSN 1424-4721) is a monthly review written by the Swiss-Prot team of the SIB Swiss Institute of Bioinformatics. Spotlight articles describe a specific protein or family of proteins on an informal tone. Follow us: Subscribe · Twitter · Facebook