By first probing the way primitive yeast make
cholesterol, a team of scientists has discovered a
long-sought protein whose human counterpart controls
cholesterol production and potentially drug metabolism.
The collaborative study by investigators at Johns
Hopkins University School of Medicine, Vanderbilt and
Indiana universities and Eli Lilly and Co., was published
in the February issue of Cell Metabolism.
The protein the researchers found is called Dap1, and
it "controls the activity of a clinically important class
of enzymes required for cholesterol synthesis and drug
metabolism," said Peter Espenshade, assistant professor of
cell biology at Johns Hopkins.
"We're excited because although we originally identified
this protein in yeast, humans not only have the same
protein, but it works the same way."
The search for Dap1 began with the hunt for factors
that influence the actions of a large family of enzymes
called cytochrome P450. These enzymes control many
life-sustaining chemical reactions in humans and other
Happily, Espenshade said, yeast have only two P450
enzymes, and both play roles in making cholesterol,
narrowing down the territory for the scientists' search and
giving them a telltale marker (the cholesterol) to
Reasoning that whatever controls the P450s likely
would be turned on and off at roughly the same time as the
P450 enzymes themselves, the researchers found that Dap1
does just that in the yeast cell.
To figure out what Dap1 does, Espenshade and
colleagues genetically altered yeast cells to lack Dap1.
Those cells predictably were unable to make cholesterol and
instead contained a buildup of cholesterol precursors.
The research team then tracked Dap1's counterpart in
humans by looking for similar proteins in a computer
database and repeated their experiments in human kidney
cells engineered to lack the human version of Dap1. As in
yeast, the altered human cells accumulated cholesterol
precursors and died because cholesterol is essential for
To show that Dap1 works directly with P450s and not
through some other biochemical steps, Espenshade's team
tested the ability of human Dap1 protein to bind to four of
the 57 known human P450 enzymes, essentially challenging
Dap1 to bind to P450s that perform totally different
functions in different cells as a way to see how
far-reaching its control might be.
Dap1 locked on to all four P450s, including one
required for clearing half of all known drugs from the
body, another involved in making bile and one required for
making natural steroid hormones in the adrenal glands.
"Collectively, our experiments suggest that Dap1 acts
as a common regulator of cytochrome P450s in mammals,"
Because Dap1 affects one particular P450 responsible
for drug metabolism, Espenshade suspects that genetic
variations in the genetic blueprint coding for Dap1 may
provide clues to how and why different people react
differently to certain drugs.
"Understanding the molecular underpinnings of
so-called pharmacogenetic variation will have a big impact
on the future of medicine," he said.
The research was funded by the National Institutes of
Health, American Heart Association and Burroughs Wellcome
Authors on the paper are Adam Hughes and Espenshade,
of Johns Hopkins; David Powell and Andrew Link, of
Vanderbilt University School of Medicine; Martin Bard, of
Indiana University-Purdue University Indianapolis; and
James Eckstein and Robert Barbuch, of Eli Lilly and Co.