StAR appears to do its best work when it takes the shape of a molten globule. That is not news from astronomers or cosmologists, but from the molecular biology lab of Walter L. Miller, MD, professor of pediatric endocrinology at UC San Francisco. Miller's group is working to find out how cells make steroid hormones, chemicals that are essential for life, for health and for reproduction because they control the body's salt balance, sugar balance and sexual function. StAR - steroidogenic acute regulatory protein -- is an unusual protein that recently has been shown to perform a key role in the steroid-making system. In a paper in the June 22, 1999 issue of the journal Proceedings of the National Academy of Sciences, Miller, UCSF postdoctoral fellow Himangshu Bose and their colleagues report that the StAR protein partially unfolds from its finished shape in order to take a shape that allows it to work inside the cell. In protein-folding jargon, a partially unfolded protein is called a "molten globule."
Cells in the adrenal and sex glands manufacture steroid hormones by changing cholesterol into related chemicals, in a process that involves many steps. For the first step, cholesterol has to be transported into mitochondria, small energy-generating spheres inside the cell. In work published over the past few years, Miller's group and their colleagues from other universities have demonstrated that StAR helps speed up this cholesterol-transfer process. These groups also have cloned the gene that controls the StAR protein, and have shown that a rare inherited disease is caused by mutations of that gene. The disease provides stark proof of the essential role of steroid hormones.
Children born with the rare disease, congenital lipoid adrenal hyperplasia (lipoid CAH) cannot make steroid hormones in part because they lack a functioning StAR protein. "Many of these children die in infancy for lack of mineralcorticoids, the hormones that regulate salt balance in the body," Miller said. "Hormone replacement therapy can save their lives, but the children also must receive mineralcorticoid and glucocorticoid replacements throughout their lives to help maintain salt and sugar balance."
Miller's lab has been working in cooperation with Michael Baldwin, PhD, and Thomas James, PhD, in the department of pharmaceutical chemistry at UCSF to determine the structure of StAR and learn how it works.
Miller credits Bose, a postdoctoral fellow with a background in physical chemistry, for enlisting help from Baldwin, James and other UCSF experts in protein spectroscopy to study how the StAR protein is folded into its final shape. Most proteins are not active in this incompletely folded state, but Bose concluded that StAR does its work of ushering cholesterol into the mitochondria when it is partially unfolded.
That may be because the ports that allow entry into mitochondria are very small, Bose said. "Think of a protein as a ball of spaghetti. It can only get into the mitochondria one strand at a time." A section of the protein called the "mitochondrial leader" enters a port. Inside the mitochondria, import machinery begins to pull the protein in - as if it were slurping in a strand of spaghetti. StAR unfolds to enter the port, but a compactly folded section of the protein unfolds more slowly and creates a pause in the process.
In their article for The Proceedings of the National Academy of Sciences, Bose, Miller and their colleagues propose that this flexible, "molten globule" conformation lowers the energy required for a channel to open in the mitochondrial membrane. In this form, StAR appears to function as an on/off switch for cholesterol flowing from the cell into the mitochondria. If so, it may be the first protein found that does its work in a molten globule state. More studies are necessary to determine StAR's exact structure and to work out the details of how it shepherds cholesterol into the mitochondria to become the raw material for steroids.
"We know that we have a very intriguing protein with a crucial function," Bose said. "This research is a start on determining how it looks and how it works."
REPORTERS' NOTE: For more information about the work of the Miller lab, see their website at http://itsa.ucsf.edu/~wlmlab/
CONTACT US FOR A DOWNLOADABLE COPY OF A SKETCH:
jbasu@pubaff.ucsf.edu or Janet Basu at 415-476-2557
HOW THE StAR PROTEIN ENTERS A MITOCHONDRION
Based on their data reported in the Proceedings of the National Academy of
Sciences, the scientists of the Walter Miller lab at UCSF hypothesize that
StAR proteins must partially unfold - becoming "molten globules" -- to
enter mitochondria. A helix-shaped segment of the protein called the
mitochondrial leader (NH2)is pulled inside the outer and inner mitochondrial
membranes (OMM and IMM). A segment that is resistant to unfolding
(63-188) slows the entry, allowing more opportunity for the biologically
active segment of the protein to interact with the outer mitochondrial
membrane. In this form, StAR appears to function as an on/off switch for
cholesterol flowing from the cell into the mitochondria - the first step in
changing cholesterol into steroid hormones.
Journal
Proceedings of the National Academy of Sciences