(right) and Sung Chang Lee
Photo courtesy of
The Scripps Research Institute
Scientists have discovered how the primary protein responsible for multidrug chemotherapy resistance changes shape and reacts to drugs.
They believe this information will aid the design of better molecules to inhibit or evade multidrug resistance.
The researchers noted that the proteins at work in multidrug resistance are ABC transporters. An important ABC transporter, P-glycoprotein (P-gp), catches harmful toxins in a “binding pocket” and expels them from cells.
The problem is that, in cancer patients, P-gp sometimes begins recognizing chemotherapy drugs and expelling them too. Over time, more and more cancer cells can develop multidrug resistance, eliminating all possible treatments.
“Virtually all cancer deaths can be attributed to the failure of chemotherapy,” said study author Qinghai Zhang, PhD, of The Scripps Research Institute in La Jolla, California.
He and his colleagues theorized that scientists might be able to design more effective cancer drugs if they had a better understanding of P-gp and how it binds to molecules.
A better look at transporters
For their first study, published in Structure, the researchers looked at P-gp and MsbA, a similar transporter protein found in bacteria, under an electron microscope. This helped them solve a major problem in transporter research.
Until recently, scientists could only compare images of crystal structures made from transporter proteins. These crystallography images showed single snapshots of the transporter but didn’t show how the shape of the transporters could change.
Using electron microscopy, however, a whole range of different conformations of the structures could be visualized, essentially capturing P-gp and MsbA in action.
The research was also aided by the development of new chemical tools. The team used a solution of lipids and peptides to mimic natural conditions in the cell membrane. They used a novel chemical called beta-sheet peptide to stabilize the protein and provide enough stability for a new perspective.
Together with electron microscopy, this technique enabled the researchers to capture a series of images showing how transporter proteins change shape in response to drug and nucleotide binding. They found that transporter proteins have an open binding pocket that constantly switches to face different sides of membranes.
“The transporter goes through many steps,” Dr Zhang said. “It’s like a machine.”
A closer look at binding
In a second study, published in Acta Crystallographica Section D, the scientists investigated the drug binding sites of P-gp using higher-resolution X-ray crystallography. And they discovered how P-gp interacts with ligands.
The researchers studied crystals of the transporter bound to 4 different ligands to see how the transporters reacted. They found that when certain ligands bind to P-gp, they trigger local conformational changes in the transporter.
Binding also increased the rate of ATP hydrolysis, which provides mechanical energy and may be the first step in the process by which the binding pocket closes.
The team also discovered that ligands could bind to different areas of the transporter, leaving nearby slots open for other molecules. This suggests it may be difficult to completely halt the drug expulsion process.
Dr Zhang said the next step for this research is to develop molecules to evade P-gp binding.
