Protein production is essential for life; cells use DNA to transcribe active genes into RNA sequences, and ribosomes translate coding RNA molecules into proteins, which happens in the cytoplasm. DNA is kept in the nucleus of the cell, so RNA molecules are usually transcribed there, and the RNA gets exported from the nucleus to the cytoplasm to be translated by ribosomes. Ribosomes are made of two large subunits, but those are typically made within the nucleus. Once the subunits exit the nucleus and enter the cell cytoplasm, they are finally assembled into ribosomes.
Researchers have now captured the passage of the larger ribosomal subunit, called pre-60s, through pores in the nuclear membrane and into cytoplasm, on video. This work, which used a specially modified microscope and fluorescent stains, has been reported in Nature Communications. This study took years of work and experimental optimization.
Each nuclear pore has a diameter of about a ten-thousandth of one millimeter, and there are a few thousand of these pores on the nucleus of every human cell. The pores are covered with a kind of gel that normally keeps anything large from moving through it. The large ribosomal subunits have to use the help of export receptors to get past the plugs.
"On the outside of each pore sits a protein gripper that pulls out the ribosome unit," explained study co-author Dr. Jan Peter Siebrasse of the University of Bonn.
These grippers limit the speed at which the subunits can get past the nuclear membrane. The study indicated that pre-60s subunits accumulate right where the gripping protein moves the subunits through the pore. Not every subunit gets through, either. In about every third case, a subunit failed to exit the pore. The movement is still successful and rapid, however; the study suggested that between 35 and 50 subunits can move through one pore in a single second.
This research has provided new insight into ribosomes, as well as opening up new ways of studying transport mechanisms. "It's not without reason that the reviewers have called our study a reference that we hope other groups can benefit from," noted corresponding study author Professor Dr. Ulrich Kubitscheck of the Institute for Physical and Theoretical Chemistry at the University of Bonn.