In order to be able to fulfil the biological functions in a cell, each newly synthesised protein has to take on a unique three-dimensional structure. Cellular stress or mutations interfere with the correct structure formation, i.e. native protein folding, and can lead to the deposition of detrimental, insoluble protein aggregates. Protein aggregation is a central problem arising through the elevated temperatures to which the cells are exposed, and is also observed in the development of neurodegenerative diseases such as Parkinson’s, Alzheimer’s or prion diseases. Prof. Bernd Bukau’s team from the Centre for Molecular Biology at the University of Heidelberg (ZMBH) has succeeded for the very first time in understanding the molecular mechanisms that lead to the disaggregation of stress-related protein aggregates in the cells.

The cellular system of protein quality control, consisting of molecular folding helpers known as chaperones, and proteases, provides for the repair or the degradation of aggregated proteins. Bukau’s team is investigating the phenomenon of protein disaggregation on the model of ClpB, a chaperone of Escherichia coli bacteria that inhabit our intestines. ClpB is an energy-dependent, spherical protein with a central channel and the unique ability to completely solubilise protein aggregates and return the individual proteins into their native state, in cooperation with another chaperone system (DnaK chaperone).

Earlier work by the team was impressively able to demonstrate that ClpB extracts individual protein molecules from the protein aggregates and subsequently threads them through a central channel, an energy-dependent mechanism that is referred to as translocation.

Disaggregation has for a long time been investigated on model proteins that unfold completely under stress conditions. However, many cellular proteins are very complex and consist of several folding domains that form mixed aggregates under stress, and which then contain a mixture of misfolded and native domains. The fate of the native domains during the disaggregation of the protein aggregates has for a long time been unclear. In a new study, published this week in the online edition of the journal “Nature Structural Molecular Biology” (DOI-No. 10.1038/nsmb.1425), the authors succeeded in clarifying the mechanism of the ClpB-mediated disaggregation of such mixed, physiologically relevant aggregates.

ClpB reactivates mixed protein aggregates rapidly and efficiently. ClpB only recognises the misfolded part of the protein and threads this into its central channel. The stable domains are not affected. The relatively rapid reactivation of aggregated proteins, whose misfolded moiety is flanked by stable domains, shows for the very first time that the ClpB-mediated disaggregation of protein aggregates does not depend on freely accessible ends of the aggregated proteins, but can start at exposed, internal segments of the misfolded structures.

The results highlight the adaptation of the ClpB chaperone system to its cellular function. ClpB catalyses the disaggregation and reactivation of aggregated proteins, something that is vital for a cell, and specialises in the recognition and unfolding of misfolded domains; the translocation of native domains would be an unnecessary waste of energy. The processes and mechanisms that lead to the aggregation of proteins and its reversal, are of medical importance since protein aggregation is associated with many neurodegenerative diseases. No ClpB homologue has so far been identified in mammals, however there is great evidence that disaggregation also occurs in higher eukaryotes. It now needs to be clarified whether the disaggregation of these proteins follows a chaperone-mediated mechanism that is similar to that described in bacteria.

Source: University of Heidelberg - 23.05.08

Further information:
Dr. Ralf Tolle
Centre for Molecular Biology
at the University of Heidelberg (ZMBH)
Im Neuenheimer Feld 282
69120 Heidelberg
Tel.: +49 (0)6221 546850
E-mail: r.tolle@zmbh.uni-heidelberg.de