Abstract
Hsp90 is a conserved chaperone that facilitates protein homeostasis. Our crystal
structure of the mitochondrial Hsp90, TRAP1, revealed an extension of the N-terminal
β-strand previously shown to cross between protomers in the closed state. In
this study, we address the regulatory function of this extension or
‘strap’ and demonstrate its responsibility for an unusual temperature
dependence in ATPase rates. This dependence is a consequence of a thermally sensitive
kinetic barrier between the apo ‘open’ and ATP-bound
‘closed’ conformations. The strap stabilizes the closed state through
trans-protomer interactions. Displacement of cis-protomer contacts from the apo state
is rate-limiting for closure and ATP hydrolysis. Strap release is coupled to rotation
of the N-terminal domain and dynamics of the nucleotide binding pocket lid. The strap
is conserved in higher eukaryotes but absent from yeast and prokaryotes suggesting
its role as a thermal and kinetic regulator, adapting Hsp90s to the demands of unique
cellular and organismal environments.
DOI:http://dx.doi.org/10.7554/eLife.03487.001
Proteins—which are made of chains of molecules called amino acids—play
many important roles in cells. Before a newly made protein can work properly, the
amino acid chain has to be folded into the correct three-dimensional shape. Many
proteins that have folded incorrectly are harmless, but some can disrupt the cell and
cause damage. Although most proteins can fold properly on their own, they are often
helped by ‘chaperone’ proteins, which speed up the process and
encourage correct folding.
Many chaperone proteins belong to a family called the heat shock proteins, which are
found in almost all species: from bacteria, to plants and animals. High temperatures
can severely impair and destabilize proper protein folding, and the heat shock
proteins counteract this by helping to prevent, or correct, protein misfolding. Most
animals and plants have at least four genes that make different versions of heat
shock protein 90 (Hsp90). These versions work in different places in the cell and
one—called TRAP1—is found in internal compartments called mitochondria.
Along with its role in assisting protein folding, TRAP1 also acts as an indicator of
the health of the proteins in the mitochondria.
One section or ‘domain’ of Hsp90 is able to bind to and break down a
molecule called ATP. This releases energy that is used to change the shape of the
protein-binding domain—which is responsible for helping other proteins to
fold. Recent studies of TRAP1 using a technique called protein crystallography
highlighted the presence of a short amino acid tail or ‘strap’ at one
end of the protein, but it is not known what role it may play in protein folding.
In this study, Partridge et al. reveal that the amino acid strap of TRAP1 controls
the breakdown of ATP in a way that depends on the surrounding temperature. Similar
straps are also present in the Hsp90 proteins that are found in other parts of the
cell. However, the strap is absent from the Hsp90 proteins of yeast and bacteria.
These experiments used proteins that had been taken from living cells and placed in
an artificial setting, so an important next step will be to study the role of the
strap in the folding of proteins inside living cells. Also, future work could
investigate the potential role of the protein in maintaining healthy
mitochondria.
DOI:http://dx.doi.org/10.7554/eLife.03487.002
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