October 18, 2010

Structure of Receptor Involved in Cancer, HIV Infection

Molecular model of CXCR4 structure Structure of a pair of linked CXCR4 molecules (blue and gold) bound by loop-shaped peptide inhibitors (red and magenta).Raymond C. Stevens, Scripps Research Institute.

Scientists determined the 3-dimensional structure of a molecule involved in HIV infection and many forms of cancer. The accomplishment sheds light on how the molecule functions. It could also point to ways of locking out HIV and stalling cancer's spread.

The molecule, CXCR4, is part of a large family of proteins called G-protein coupled receptors. These proteins span the cell's membrane and transmit signals from the external environment to the cell's interior. They help control practically every bodily process, including cell growth, hormone secretion and light perception. Nearly half of all drugs on the market target these receptors.

CXCR4 has been linked to more than 20 types of cancer. Normally, it helps regulate the immune system and stimulate cell movement. But when the signals that activate CXCR4 aren't properly regulated, the receptor can spur the growth and spread of cancer cells. In 1996, a team of researchers at NIH discovered that CXCR4 also acts as a co-receptor for HIV—the virus that causes AIDS—to enter cells (along with the primary receptor, CD4).

A research team led by Dr. Raymond C. Stevens of the Scripps Research Institute set out to shed light on how CXCR4 functions by capturing snapshots of the protein 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. The study received support from 2 major NIH initiatives: the structural biology program of the NIH Common Fund and the Protein Structure Initiative.

Determining the structure of CXCR4 represented a major challenge because membrane proteins are notoriously tricky to coax into the crystal form required for X-ray crystallography. After 3 years of optimizing conditions for producing, stabilizing and crystallizing the molecule, the scientists finally generated 5 distinct structures of CXCR4. Their findings appeared in the October 7, 2010, advance online edition of Science.

The structures show that CXCR4 molecules form closely linked pairs, confirming evidence from other experiments 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 show that CXCR4 is shaped like 2 white wine glasses touching in a toast, with the inhibitors bound at the sides of the bowls. By detailing these contacts, the researchers say 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.

"Scientists have been studying CXCR4 for years but have only been able to guess at what it looks like," says NIH Director Dr. Francis S. Collins. "Now that we have its structure, we have a much clearer picture of how this medically important molecule works, opening up entire new areas for drug discovery."

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