"A molecular switch is a molecule, in this case a protein, that can switch from one state to another based upon some external signal," says Marc Ostermeier, PhD, assistant professor, chemical and biomolecular engineering, Johns Hopkins University, Baltimore, Md. "The protein can exist in two different states depending on whether a signal is present or not. Here the state of enzymatic activity is either more active or less active."
For the proof-of-principle experiment Ostermeier's team picked two proteins that were easy to work with and easy to tell if they were active or inactive. The proteins, TEM-1 beta-lactamase (BLA) and the E. coli maltose binding protein (MBP) are functionally unrelated. The researchers aimed to bind these two proteins to create a switch that couples the rate of beta-lactam hydrolysis to maltose binding and concentration.
"A switch requires two functions, the function you want to control, which, in this case, was modulating the activity of the beta-lactamase enzyme by turning it on or off," says Ostermeier. "The other function is something that is going to recognize the signal to tell the enzyme to turn on or off. Our signal recognition protein is the maltose binding protein. When the protein binds maltose then that conformational change affects the beta-lactamase activity."
Because these two proteins don't normally interact they wanted to covalently bind them to each other. The genes of the two proteins are fused together in such a way so that the proteins fold in two separate domains but are covalently linked. "People have taken two different proteins and fused them together to try to make switches where one depends on another with varying degrees of success," says Ostermeier. What is different about their method, he says, is that they produce thousands of variations of fusion geometries to find the one that's ideal.
"We want to have the correct connection and geometric rearrangement so that the signal that maltose is present propagates through a conformation change in the protein into the beta-lactamase domain and affects its activity," says Ostermeier.
Ostermeier believes the molecular switch technique could be used to create sensors that could detect the presence of foreign agents. They also have the potential to be used to detect diseased cells and deliver drugs or for gene-based therapies where a signal could be used to turn on the expression of a therapeutic protein.
By Elizabeth Tolchin