Molecular Motors in Protein Folding
The focus of our research is a particular type of molecular motor proteins, the so-called AAA+ proteins (A
ssociated with a variety of cellular a
ctivities). As their name indicates, AAA+ proteins are involved in a number of important cellular processes including protein degradation, protein disaggregation, DNA replication, and membrane fusion. Despite their functional heterogeneity, they share a large degree of sequence homology and an evolutionary conserved mechanism. They consist of one or two conserved ATP binding modules linked to function-specific auxiliary domains. Each AAA+ module harbors canonical Walker A and Walker B sequence motifs that are critical for nucleotide binding and hydrolysis, respectively. In many cases, AAA+ proteins assemble into ring-shaped hexamers enclosing a central channel. AAA+ proteins are considered as molecular motors because they use chemical energy provided by ATP hydrolysis to catalyze force-induced structural rearrangements of client molecules.
Functional models of Hsp104 (left) and Hsp93 (right). Hsp104 uses the energy from ATP hydrolysis to disrupt the non-covalent interactions between polypeptide chains trapped in protein aggregates, resulting in their release and renaturation. Hsp93 is believed to associate with components of the TIC complex in the chloroplast stroma, which constitutes a part of the translocation pore connecting chloroplast and cytosol. In this case, ATP hydrolysis is used to drive polypeptide transport across the double membrane surrounding the chloroplast.
Using a predominantly biochemical approach, we are investigating how the two AAA+ proteins Hsp104 and Hsp93 achieve their particular cellular function using the same type of molecular framework (Figure). Both Hsp104 and Hsp93 are class-1 AAA+ proteins, i.e. they contain 2 AAA+ modules per subunit and a total of 12 AAA+ modules in the functional hexamer. Hsp104 is a molecular chaperone from yeast that catalyzes the disassembly of protein aggregates
in co-operation with the Hsp70/40 chaperone system. Hsp93 is a protein localized in the stroma of plant chloroplasts, to which two different functions have been attributed. The soluble fraction of Hsp93 is believed to serve as an unfolding unit for the stromal ClpP protease, while the membrane-associated fraction of Hsp93 has been implicated in catalyzing the import of polypeptides
from the cytosol into the chloroplast. We concentrate our efforts on understanding the latter function. Click on the cartoons to learn more about the two projects