A research team at the Max Planck Institute of Biochemistry , Martinsried/Germany, and the Institute of Microbiology, University of Regensburg, has solved the crystal structure of the thermosome. This structure, published in Cell on April 3, 1998, provides clues for understanding the mode of action of archaeal and eukaryotic chaperonins.
Proteins require a defined three-dimensional structure to
fulfill central tasks in living organisms. Protein folding -
the process in which they acquire their native structure -
has been recognized as a spontaneous process. The
three-dimensional structure is coded in the sequence of the
building blocks, the amino acid. However, in the cellular
environment proteins have to adopt their structure in a very
short time after synthesis in the presence of a large number
of other cellular components that could interfere with this
process. Therefore a class of proteins, the chaperones, has
evolved that rescues the folding intermediates from
aggregation. A special kind of chaperones are double-ring,
oligomeric containers, termed chaperonins. They provide
closed compartments that shield folding proteins from the
cellular environment.
According to their general importance chaperonins are present
in all three kingdoms of life: eubacteria, archaea and
eukaryotes. Based on their evolutionary relationship they
fall into two distinct groups. The prototype of group I
chaperonins is represented by GroEL which occurs in the
eubacterium Escherichia coli. Group II chaperonins are
comprised of the archaeal chaperonins, e.g. the thermosome of
the present study, and the eukayotic CCT which occurs in
higher organisms as human beings.
Members of both groups consume energy to drive the folding
cycle by hydrolyzing ATP and share a basic principle of
action. However, the implementation at the molecular level
seems to differ considerably as only group I chaperonins
depend on a second, smaller kind of protein, the
cochaperonins 10, termed GroES in E. coli. These
helper molecules are not present in the group II system.
Whereas detailed structural studies were available for group
I chaperonins, including the atomic model of the GroEL-GroES
complex, such structural information was not available for
members of group II. The crystal structure of the thermosome
provides clues for understanding the mode of action of group
II chaperonins on a molecular basis, containing a built-in
lid domain that substitutes for a cochaperonin.
Journal
Cell