Cellular Protein Folding – Structure, Function and Evolution of Chaperonins

Group Leader: Dr. Jörg Martin

The long-term goal of our research is to inderstand how proteins attain their correctly folded structure within the cell. Successful protein folding requires the assistance of molecular chaperones. Members of this diverse group of proteins can prevent protein aggregation and promote folding of proteins to their native state. Many molecular chaperones are heat-shock proteins, reflecting the increased need for folding assistance in the cell under stress conditions. We use a variety of techniques to analyse structure, function and evolution of an important chaperone sub-group, the chaperonins.
 
Why do proteins need assistance at all if the information for their structure is given in their own sequence? Cells synthesize impressive amounts of proteins in a very short time, in a cytosol where the macromolecular concentration can reach more than 300 g/l. Under these conditions, the association among interacting proteins is enormously increased. Chaperone networks help to direct nascent polypeptides favourably to productive folding, and/or multimeric assembly, without aggregation.

Chaperonins, a sub-family of chaperones with ATPase activity, achieve this by shielding a single folding protein from the bulk solution inside their own structure. We employ biochemical, biophysical and genetic approaches to investigate and compare properties of chaperonins from all kingdoms; the bacterial GroEL/GroES system, the eukaryotic cytosolic CCT and the archaeal Mm-cpn. The hallmark feature of chaperonins is their double-ring cylinder form with 7-9 subunits per ring, which can enclose a polypeptide chain inside for folding. Two solutions have emerged in evolution to keep this folding cage temporarily closed. In bacteria, the ring-shaped cofactor GroES binds on top of a GroEL cylinder to occlude the cavity. In contrast, CCT and Mm-cpn have a built-in lid in form of flexible extensions lining the cavity opening.

Figure 1: The reaction cycle of the bacterial GroEL/GroES chaperonin system. The molecules are shown
in cross-sections. Each GroEL subunit consists of a central domain (red) an apical substrate-binding
domain (dark green) and a connecting intermediate region (yellow). The binding of unfolded substrate
protein (light blue) to one GroEL ring is followed by binding of ATP and the cochaperonin GroES (light
green). The substrate is released into the now-closed ring cavity where it can fold. Following ATP
hydrolysis, the binding of ATP to the opposite ring triggers the dissociation of GroES and the dissociation
of folded substrate protein from the complex.

The eukaryotic CCT is related to the homo-oligomeric archaeal Mm-cpn, but has evolved eight different subunits to form its rings. In addition to interacting with a variety of substrate proteins, CCT has a central and essential role in the folding of actin and tubulin, which bind to distinct CCT subunits. The expansion of different CCT subunit genes in evolution occurred early in the eukaryotic lineage. This diversification is thought to be linked to the rapid development of the cytoskeleton which took place around the same time. One major challenge is to understand how CCT can accommodate actin and tubulin so specifically, while at the same time still serving as a general chaperonin like its archaeal relatives. Additionally, the mechanism of action of all chaperonin systems awaits detailed analysis. To this aim, we recently developed a chaperonin model system from the methanogen Methanococcus maripaludis that allows us to compare the sophisticated properties of CCT with those of its ancestral counterparts.

Figure 2: Model for the reaction cycle of the archaeal Mm-cpn chaperonin system. The molecules are shown in cross-sections. Each Mm-cpn subunit consists of a central domain (red) an apical substrate-binding domain (dark green) and a connecting intermediate region (yellow). The binding of unfolded substrate protein (light blue) to one Mm-cpn ring is followed by binding of ATP. This event induces conformational changes in helical domain protrusions (light green), leading to closure of the cylindrical cavity. The substrate folds inside the chaperonin complex and dissociates from it after ATP hydrolysis has occurred.

Presently, our group focusses on the following aspects of Chaperonin research:

• Characterization of GroEL/GroES chaperonin systems from various bacteria with an emphasis on the structure and function of unusual GroES variants
• Mechanistic in vitro analysis of the archaeal Chaperonin system Mm-cpn and its accessory factor GimC from Methanococcus maripaludis

Recent Publications

Open positions:

We currently have positions for graduate (PhD) and undergraduate (Diploma, HiWi) students, who should have a strong background in biochemistry.