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Frauke Melchior and the SUMO wrestling match between proteins

Fifteen years ago, molecular biologist Frauke Melchior discovered a new mechanism of posttranslational protein modification that controls a variety of processes in eukaryotic cells. A small protein called SUMO is covalently bound to target proteins by specific enzymes and cleaved by other enzymes. This discovery has shaped Melchior’s scientific career.

Professor Dr. Frauke Melchior © ZMBH

“SUMOylation Coming of Age“ is the title of a review article written by molecular biologist Professor Dr. Frauke Melchior of the Centre of Molecular Biology (ZMBH) at the University of Heidelberg and her colleague Dr. Annette Flotho for publication in the renowned journal Annual Review of Biochemistry. The article is due to be published in June 2013. 

Almost fifteen years ago Melchior made the discovery that has shaped her scientific career. She discovered a new mechanism of posttranslational protein modification: the reversible covalent attachment of a small protein called SUMO to other proteins, which can alter their function in many ways. 

The discovery of SUMO

Conjugation and deconjugation of SUMO. © Nature Reviews Molecular Cell Biology
Frauke Melchior studied chemistry and gained her PhD in chemistry from the University of Marburg in 1990. She then worked for two years as postdoctoral researcher at the Max Planck Institute for Biophysical Chemistry in Göttingen in the group of Volker Gerke (now at the University of Münster) who introduced her to the field of molecular cell biology. Her interest in the transport of macromolecules developed from her work with the pore complex of the nuclear membrane. This led her to Larry Gerace’s laboratory at the Scripps Research Institute in La Jolla, California, where Gerace had just developed a new in vitro transport assay and proved to be a very generous mentor. Melchior is convinced that the decision to go to Gerace’s laboratory was the best choice she could have made. In Gerace’s laboratory, Melchior identified the enzyme GTPase Ran in mammalian cells as playing a critical role in the transport of cargo across the nuclear membrane through the nuclear core complex. The enzyme is activated by the protein RanGAP. Using Western blotting experiments (ed. note: Western blotting uses gel electrophoresis to separate proteins) involving antibodies against the target protein (in this case RanGAP), Melchior discovered that one form of RanGAP migrated at the expected size (70kDa) on the gel and another one migrated 20 kDa larger, at 90 kDa. Sequencing analysis showed that the larger band contained the RanGAP sequence as well as a peptide tail of 101 amino acids. Melchior carried out a BLAST search and did not find any similarities with peptides other than ubiquitin, although the similarity with ubiquitin was rather low.
SUMO wrestler © JapanFestivalBerlin 2013

Melchior was able to put her solid biochemistry knowledge to good use in her research. Ubiquitin was discovered in 1977. It occurs ubiquitously (hence its name) in all eukaryotic organisms in virtually unchanged form and labels proteins for destruction. The marking of a protein with ubiquitin requires energy in the form of ATP. In analogy to the posttranslational protein modification process, the discovery of which the researchers Ciechanover, Hershko and Rose were awarded the Nobel Prize in Chemistry in 2004, Frauke Melchior carried out what she calls “the luckiest experiment of my life”. She used recombinant 70 kDa RanGAP, mixed it with the cell extract and ATP and ran a Western blot to see whether her assumption was correct. It turned out that it was; the researchers had converted the 70-kDa form into the 90-kDa form. A new protein modification was discovered and Frauke Melchior had found her key research object, which she continued to work on when she returned to Germany – first as the head of a group of researchers at the Max Planck Institute for Biochemistry in Martinsried and later as professor of biochemistry at the University of Göttingen. In 2008, she accepted the chair of Molecular Biology at the ZMBH. In the same year, she was elected member of the European Molecular Biology Organisation (EMBO). 

A mechanism for numerous processes

Melchior called the new peptide “small ubiquitin-related modifier”, SUMO for short. This abbreviation turned out to be an excellent publicity for Melchior and her team’s research, as all reports on the discovery used the catchy image of Japanese Sumo wrestlers to describe what the new protein did. “SUMO wrestles other proteins” is just one of several headlines used. It was a rare stroke of luck that Melchior had chosen to examine the RanGAP system. One major finding was that the SUMOylated protein remained stable and was easy to examine. SUMO modification is removed by isopeptidases in most proteins and can only be identified in the presence of isopeptidiase inhibitors. However, RanGAP remains modified by SUMO when cells are lysed in the absence of such inhibitors.

The conjugation and deconjugation of SUMO follows a similar mechanism to the marking of a protein with ubiquitin: an isopeptide bond is established between a glycine at the C-terminal end of SUMO and the ε amino group of a target protein’s lysine in a reaction cascade involving many enzymes. The energy required for SUMOylation is provided by an ATP-dependent activating enzyme (E1). SUMO becomes bound to the E1 enzyme and is then passed to an ATP-dependent E2 enzyme, which is a conjugating enzyme. Finally, an E3 ligase attaches it to the protein. In contrast to ubiquitin, SUMO is not used to label proteins for destruction. Posttranslational modification with small ubiquitin-related modifier (SUMO) proteins has been recognised as a key regulatory protein modification in eukaryotic cells. Hundreds of proteins and a confusingly large number of metabolic pathways such as chromatin organisation, transcription, DNA repair, the formation of multiprotein complexes, trafficking and signal transduction are subject to reversible SUMOylation. Melchior points out that it is hard to predict what SUMO modification will do to a given protein without looking at every possibility.

Parkinson’s disease is characterised by synuclein clusters © University of Graz

It is hence not surprising that disease links are beginning to emerge. For instance, research has shown that alpha synuclein, which clusters into fibrils in Parkinson’s disease, is SUMOylated upon overexpression. In cooperation with Dr. Jochen Weishaupt and his team at Göttingen University Hospital, Melchior found that non-modified alpha synuclein formed fibrils while the SUMOylated protein remained soluble. They also found that only 10 percent of the molecules had to be SUMOylated in order to slow down fibril formation. Perhaps this finding opens up new therapeutic options for the treatment of Parkinson’s disease. A further medical aspect is being examined in cooperation with Dr. Stefan Herzig from the German Cancer Research Center in Heidelberg: the SUMOylation of important regulatory proteins in metabolic diseases such as diabetes. 

Member of DFG Senate

Frauke Melchior has been a member of the German Research Foundation’s (DFG) Senate Committee for Research Training Groups since 2004. In July 2012, she was elected member of the Senate of the DFG where she represents the cell and developmental biology sector. She is also the vice-dean for research in the Faculty of Biosciences at the University of Freiburg. Despite her memberships of numerous boards, Melchior is mainly active in the field of basic research. She is involved in the CellNetworks cluster of excellence in Heidelberg where her team is specifically focused on the protein RanBP2, an E3 ligase that binds to the SUMOylated RanGAP protein discovered more than 15 years ago. RanBP2 is a component of the nuclear pore complex and plays a key role in nuclear transport and mitosis. It is highly likely that there are many other unknown E3 ligases and isopeptidases that exert an effect on metabolic pathways by way of SUMO conjugation or deconjugation. Frauke Melchior is particularly interested in the regulation of SUMOylation by reactive oxygen molecules. Oxidative stress, heat shock and other types of stress can lead to different SUMOylation patterns. This might be due to SUMO-regulating enzymes which coordinate the SUMOylation of many different processes as a response to stress.

References:
Flotho A., Melchior F: SUMOylation Coming of Age. Annual Review of Biochemistry, Vol. 82 (2013)
Frauke Melchior: How SUMO wrestles other proteins. J. Cell Biol. 187 (5), 586-587 (2009)

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