Bacteria probably aren't alone in using MDR pumps it is likely that there are probably many as yet unidentified such proteins lurking in membranes of all kinds of cells, says Sikic. The latest estimates say 2,000 of the approximately 100,000 human genes are included in the family of proteins to which the MDR pump also belongs, says Sikic. Only a fraction of those genes and the proteins they encode have been studied, he says.
While scientists expect to discover many more MDR-like proteins, they all are puzzled by the broad spectrum of chemicals, called substrates, that the pumps repel. The list of such substrates includes a surprisingly wide variety of compounds, from naturally occurring substances such as alkaloids in plants and antibiotics in dirt, to man-made chemicals such as dyes and some pesticides. Scientists have found only one common thread: all the substances repelled by MDR pumps are relatively hydrophobic (they don't dissolve easily in water).
Scientists are consequently coming to believe that there are likely a variety of pumps, each dealing with a group of its own, similar substrates. But researchers such as Lewis also suspect that an as yet unidentified natural chemical substrate has kept the pumps around throughout evolution.
Meanwhile, researchers around the globe continue to search by land and by sea for the MDR signature: indeed, bottom-dwelling flounder are among the most recent club members. As scientists better understand how and why cells universally use MDR pumps, they may well be able to beat nature at its own game.
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