The many guises of a chaperone
Proteins are one of the essential building blocks of all cells. They are responsible not only for the precise structure of cells, they also regulate a wide variety of functions, including inter- and intra-cellular transport of materials and the chemical transformations that occur within cells. In order to carry out their diverse functions, proteins have to fold into specific three-dimensional shapes. In a test tube most proteins can do this spontaneously, but in the cell the majority of them need help. A special class of proteins, known as molecular chaperones, interact with freshly made, unfolded proteins, screening them from undesirable influences (like debutantes at their first ball), and ensuring that they find their correct form. Members of the so-called Heat Shock Protein 70 (HSP70) family are particularly versatile. For example, Ssc1, which exists in the mitochondria, participates not only in protein folding and protection against protein aggregation but also transports proteins through the membrane systems of the mitochondria.
To facilitate folding of its unfolded substrates, Ssc1 acts together with accessory proteins, and itself goes through a cyclical series of conformational changes. Relatively little has so far been learned about the structural changes that occur during this cycle. In collaborative work groups led by Dr. Dejana Mokranjac of the Adolf-Butenandt-Institute and Professor Don C. Lamb of the Department of Chemistry at LMU Munich now succeeded in capturing the dynamics of the Ssc1 cycle. The different shapes adopted by Ssc1 enable it to bind distinct regions of a newly synthesized protein in turn, helping the substrate to find its correct form. The shape changes are driven by the energy released when Ssc1 binds adenosine triphosphate (ATP) and converts it into adenosine diphosphate (ADP), releasing one molecule of phosphate.
To monitor these changes, Lamb and Mokranjac and their teams introduced fluorescent groups at defined positions in Ssc1. Using the technique of fluorescence spectroscopy, it is possible to measure the distance between marker pairs – and how it changes during the functional cycle. “Interestingly, we found that, in the ATP-bound state, the conformation of the chaperone was surprisingly homogeneous and stable”, reports Lamb. “With ADP, on the other hand, the protein is much more flexible and dynamic and is able to adopt a number of distinct forms.“ This flexibility presumably allows Ssc1 to modulate the conformation of its substrates. It also has repercussions for its other function: Ssc1 is required for the import of proteins synthesized in the cytoplasm through the inner membrane of the mitochondrion.
“Ssc1 is absolutely essential for cell viability“, explains Dejana Mokranjac. “Further insights into the dynamics and functions of Ssc1 and other Hsp70-type chaperones will help us to study how errors in these processes give rise to disease states.“ Defects in protein folding are thought to play a role in the development of conditions such as cancer, and Alzheimer’s and Parkinson’s diseases. “Working out how chaperones carry out their diverse functions is important for understanding a set of processes that are indispensable for life”, says Lamb. In future studies, the researchers hope to apply fluorescence spectroscopy to study the interactions of other chaperones with substrates that need to be tickled into shape and how Hsp70 chaperones enable transport of proteins across membranes.
The work on Ssc1 was carried out under the auspices of two Clusters of Excellence: the Nanosystems Initiative Munich (NIM) and the Center for Integrated Protein Science Munich (CIPSM) and the SFBs 594 and 749.
”The Conformational Dynamics of the Mitochondrial Hsp70 Chaperone”;
Koyeli Mapa, Martin Sikor, Volodymyr Kudryavtsev, Karin Waegemann, Stanislav Kalinin, Claus A.M. Seidel, Walter Neupert, Don C. Lamb, Dejana Mokranjac;
Molecular Cell, vol 38, pp 1-12., 9 April 2010;
Professor Don C. Lamb and Dr. Dejana Mokranjac
Department of Chemistry and Adolf-Butenandt-Institute, LMU Munich
Phone: +49 (0) 89 / 2180-77564
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