While a molecule called CD4 is the primary receptor for HIV, CD4 is not sufficient for the virus to penetrate cells. In 1996, a team of researchers at NIH's National Institute of Allergy and Infectious Diseases (NIAID) discovered that CXCR4 acts as a co-receptor by helping HIV enter cells.
Normally, CXCR4 helps activate the immune system and stimulate cell movement. But when the signals that activate the receptor aren't properly regulated, CXCR4 can spur the growth and spread of cancer cells. To date, CXCR4 has been linked to more than 20 types of cancer.
The Scripps Research scientists set out to shed light on how CXCR4 functions by capturing snapshots of the protein by using a structure determination method called X-ray crystallography. To understand how natural molecules might bind and signal through the receptor and to see how potential drugs could interact with it, they examined CXCR4 bound to known inhibitors of its activity.
Determining the structure of CXCR4 represented a major challenge because membrane proteins are notoriously tricky to coax into the crystal form required for the X-ray technique. After three years of optimizing conditions for producing, stabilizing and crystallizing the molecule, the scientists finally generated five distinct structures of CXCR4.
The structures showed that CXCR4 molecules form closely linked pairs, confirming data from other experiments indicating that pairing plays a role in the proper functioning of the receptor. With this knowledge, scientists can delve into how the duos might regulate CXCR4's activity and better understand how CXCR4 functions under normal and disease conditions.
The images also showed that CXCR4 is shaped like two white wine glasses touching in a toast, with the inhibitors bound at the sides of the bowls. By detailing these contacts, the researchers said the pictures suggest how to design compounds that regulate CXCR4 activity or block HIV entry into cells. If developed into drugs, such compounds could offer new ways to treat HIV infection or cancer.
"An approach to determining protein structures that was developed with support from the NIH Common Fund and the PSI is now paying huge dividends," said Jeremy M. Berg, Ph.D., director of the National Institute of General Medical Sciences, which supports the PSI. "It illustrates how technical progress provides a foundation for rapid advances, and it also showcases the benefits of collaborations between structural biologists and scientists working in other fields for addressing fundamentally important problems with tremendous potential for medical applications."
This model shows how HIV, in gray, might latch on to immune cell receptor molecules, allowing the virus to enter and infect the cell. The viral protein, gp120, shown in light blue, binds to receptors CD4 and CXCR4, shown in tan and dark blue. Models like this one allow scientists to test their ideas about how HIV gains access to cells -- and help pinpoint important targets for developing drugs to impede HIV.
(Photo Credit: Animation courtesy of Gye Won Han of the Scripps Research Institute. Video produced by the National Institute of General Medical Sciences, part of the National Institutes of Health.)
This is a structure of a pair of linked CXCR4 molecules (blue and gold) bound by loop-shaped peptide inhibitors (red and magenta).
(Photo Credit: Raymond Stevens, the Scripps Research Institute, La Jolla, Calif.)