Rush hour never ceases at the nucleus’ border. Gene products begin their lives in the nucleus as strands of mRNA that ship out into the cytoplasm, where they serve as templates for protein synthesis. Many of these proteins, such as transcription factors, subsequently return to sender, crossing back into the cell’s central organelle. To traverse the nuclear envelope, proteins must transit through a nuclear pore complex (NPC). This protein channel serves as a gatekeeper to the nucleus, restricting passage to select proteins that carry a nuclear localization signal (NLS)—an amino acid sequence that stamps the protein for delivery to the nucleus.
In a recent study published in Nature Physics, researchers found that proteins harboring a flexible domain near the NLS enter the nucleus faster. To mimic this limber protein element, the biophysicists designed a bendable protein tag that expedited delivery of protein cargo into the cell’s core organelle.
“People are certainly looking at how to deliver various therapeutics, diagnostics, or just simply research tools to the nucleus, and this could be rather important in very significantly improving the efficiency of that process,” said Michael Rout, a cell biologist at the Rockefeller University who was not involved with the work.
Scientists previously found that the NPC shapeshifts to allow cargo to cross the nuclear threshold, but scientists know little about how structural alterations to the protein parcels themselves affect transport. “Cargoes have been to some degree seen as like the corpse at a funeral—they’re the purpose for the whole ceremony, but they don’t take an active part in the process,” said Rout.
To study the relationship between a protein’s molecular makeup and its movements, Sergi Garcia-Manyes, a biophysicist at the Francis Crick Institute and study coauthor, developed a system to time protein transport into the nucleus. He and his team chose a common protein motif called the immunoglobulin domain (Ig) as their test subject. They worked with two Ig mutants: one that was more flexible and another that was more rigid compared to wild type Ig. However, Ig domains don’t have an NLS, so the researchers gave each version the nuclear postage stamp. Fusing these mutant constructs to a fluorescent protein allowed the researchers to monitor protein distribution under a microscope and time their shipments to the nucleus. The researchers were ready to take their souped-up proteins to the races. When they pitted the mutants against one another, they found that the flexible Ig domain took less time to enter the nucleus than the stiff variety.
By Kamal Nahas, PhD
Article can be accessed on: The Scientist