The MDR protein got its name from the place
it was originally discovered roughly a decade ago: in the membranes of cancer
cells. A huge, snake-like protein, the MDR pump threads through a membrane
like a needle through a piece of cloth, back and forth across the membrane
a dozen times. The pump uses packets of energy made by the cell, called
ATP, to force out toxins.
Biologist Epel agrees that MDR pumps in worms, mussels and humans might
all have natural substrates. While the primordial role for these proteins
may have been to protect animals from natural toxins, he suggests, MDR pumps
might also transport "normal" substances, such as steroids and
some fats, out of cells.
For instance, yeast cells use MDR-like proteins to pump out hormones involved
in mating, he says. Yet, in mammals, MDR pumps are positioned so as to encounter
potentially harmful substances face-to-face in important places like
the intestine, the placenta, and the blood-brain and blood-testes barriers,
making them an excellent first line of defense, says Epel.