"The ribosome with the membrane and the other components is a simulation of over 3 million atoms," Schulten said, a feat accomplished with powerful computers and "over 20 years of experience developing software for modeling biomolecules." (Schulten is principal investigator of the NIH-funded Resource for Macromolecular Modeling and Bioinformatics at Illinois, which supports the study of large molecular complexes in living cells, with a special focus on the proteins that mediate the exchange of materials and information across biological membranes.)
This analysis found that regions of the membrane channel actually reach into the ribosome exit to help funnel the emerging protein into the channel. Depending on the type of protein being built, the channel will thread it all the way through the membrane to secrete it or, as in this case, open a "side door" that directs the growing protein into the interior of the membrane, Schulten said. The researchers also saw for the first time that the ribosome appears to interact directly with the membrane surface during this process.
The researchers found that a signal sequence at the start of the growing protein threads through the channel and anchors itself in the membrane. Previous studies suggested that this signaling sequence "tells" the ribosome what kind of protein it is building, directing it to its ultimate destination inside or outside the cell.
"This new work visualizes this process for the first time, giving researchers the first image of how nascent proteins actually get into membranes," Schulten said. "It's like going to Mars and being the first to look at Mars."
In a second study, in the Proceedings of the National Academy of
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign