After four years of hard work Croatian scientists Professor Ivan Dikic and Koraljka Husnjak with the help of multinational team solved a mystery that puzzled scientists for 30 years.
Professor Ivan Dikic solved "Nobel mystery"Staff writer ▼ | Saturday May 24, 2008 3:50AM ET
The basic mechanism of waste disposal in human cells was discovered by Aaron Ciechanover, Avram Hershko, and Irwin Rose, and they won Nobel Prize in Chemistry in 2004. Although it was known from their work that proteins that are to be removed from the cells are marked with ubiquitin molecules, it was not known precisely how that mechanism works.
The thing called proteasome is used to degrade old or no longer needed proteins. That way our boy stays clean and functions normally. On the other hand, if that systems is malfunctioning a disease may occurs, the examples are Alzheimer's or Parkinson's disease. And that's where Mr. Dikic and Ms. Husnjak stepped in: They found a receptor for ubiquitin on the proteasome which may have a key role in fighting tumors.
Scientist at Frankfurt University with their friends, an international team of scientists, reported a breakthrough which reveals how that mechanism works. In the current edition of the scientific journal "Nature" they report their findings.
"A discovery of this kind happens only once in a researcher's lifetime" said Professor Dikic, the leader of the group of scientist at the Institute for Biochemistry. The importance of their work is illustrated by the fact that editors of "Nature" have accepted two papers describing this discovery: an article (leading manuscript in the issue,) and a letter (regular publication).
However, things were looking very different only a year ago when it appeared that the research groups involved in this project were treading water. The scientists were hoping to solve structure of the portal protein from yeast using protein crystallography but the protein refused to crystallize. However, Koraljka Husnjak, a postdoctoral researcher found a way to isolate the ubiquitin binding domain in the mammalian protein, that was amenable for rapid crystallization and subsequent determination of its structure.
On the upper side of the proteasome there is a kind of gatekeeper's lodge with a narrow entrance leading to an inner chamber, where aggressive enzymes cleave the protein. But first the protein is subjected to a strict control procedure to ensure that it is indeed destined for the shredder. If the gatekeeper (a receptor) recognizes that the protein is tagged with ubiquitin, the tagged protein is unfolded and can then pass through the narrow entrance. While this takes place the ubiquitin separates from the protein ready to be re-used. Until now, only one such gatekeeper was known, a proteasomal receptor called Rpn 10.
The scientists then genetically removed Rpn 10 from the cell and they discovered that the proteasome continued to function normally. The conclusion: There must be an additional protein in the cell, which works similarly as Rpn 10. This has now been discovered: protein Rpn 13.
According to Koraljka Husnjak the first breakthrough occurred about four years ago, when they found out that ubiquitin binds to a subunit in the gatekeeper's lodge. "So it became clear to us that the proteasome subunit might act as ubiquitin receptor on the proteasome. But first of all we had to clarify this binding site's function and understand the details of the binding process at an atomic level". Ivan Dikic then asked other leading international groups for their expertise in helping to solve this complex research problem.
The X-ray structural analysis was carried out by Prof. Michael Groll and his group at the Technical University in Munich, and a group led by Prof. Kylie Walters at the University of Minnesota, Minneapolis undertook the NMR structure work. As soon as the binding mechanism had been understood at an atomic level, Professor Finley and his group at Harvard Medical School conducted experiments with various yeast strains in which they were able to prove that in living cells the process was indeed identical to that already suggested by the structural model. ■