Chemical bonds within the eye-lens protein gamma-B
crystallin hold the protein together and are therefore important for the
function of the protein within the lens. Contrary to previous assumptions, some
of these bonds, called disulphide bridges, are already formed simultaneously with
the synthesis of the protein in the cell. This is what scientists at Goethe
University Frankfurt, Max Planck Institute of Biophysics and the French Institute de Biologie
Structurale in Grenoble have discovered.
FRANKFURT. The
lens of the human eye gets its transparency and refractive power from the fact
that certain proteins are densely packed in its cells. These are mainly
crystallines. If this dense packing cannot be maintained, for example due to
hereditary changes in the crystallines, the result is lens opacities, known as cataracts,
which are the most common cause of vision loss worldwide.
In order for crystallins to be packed
tightly in lens fibre cells, they must be folded stably and correctly. Protein
folding already begins during the biosynthesis of proteins in the ribosomes, which
are large protein complexes. Ribosomes help translate the genetic code into a
sequence of amino acids. In the process, ribosomes form a protective tunnel
around the new amino acid chain, which takes on three-dimensional structures
with different elements such as helices or folded structures immediately after
the tunnel's formation. The gamma-B crystallines studied in Frankfurt and
Grenoble also exhibit many bonds between two sulphur-containing amino acids,
so-called disulphide bridges.
The production of these disulphide bridges is not easy
for the cell, since biochemical conditions prevail in the cell environment that
prevent or dissolve such disulphide bridges. In the finished gamma-B
crystalline protein, the disulphide bridges are therefore shielded from the
outside by other parts of the protein. However, as long as the protein is in
the process of formation, this is not yet possible.
But because the ribosomal tunnel was considered too
narrow, it was assumed - also on the basis of other studies - that the
disulphide bridges of the gamma-B crystallins are formed only after the
proteins have been completed. To test this assumption, the researchers from
Frankfurt and Grenoble used genetically modified bacterial cells as a model
system, stopped the synthesis of the gamma-B crystallins at different points in
time and examined the intermediate products with mass spectrometric, nuclear
magnetic resonance spectroscopic and electron microscopic methods, and supplemented
these with theoretical simulation calculations. The result: The disulphide
bridges are already formed on the not yet finished protein during the synthesis
of the amino acid chain.
"We were thus able to show that
disulphide bridges can already form in the ribosomal tunnel, which offers
sufficient space for this and shields the disulphide bridges from the cellular
milieu," says Prof. Harald Schwalbe from the Institute of Organic
Chemistry and Chemical Biology at 51猎奇. "Surprisingly,
however, these are not the same disulphide bridges that are later present in
the finished gamma-B crystallin. We conclude that at least some of the
disulphide bridges are later dissolved again and linked differently. The reason
for this probably lies in the optimal timing of protein production: the
'preliminary' disulphide bridges accelerate the formation of the 'final'
disulphide bridges when the gamma-B crystallin is released from the
ribosome."
In further studies, the researchers now
want to test whether the synthesis processes in the slightly different
ribosomes of higher cells are similar to those in the bacterial model system.
Publication: Linda Schulte, Jiafei
Mao, Julian Reitz, Sridhar Sreeramulu, Denis Kudlinzki,
Victor-Valentin Hodirnau, Jakob
Meier-Credo, Krishna Saxena, Florian Buhr, Julian D. Langer, Martin Blackledge,
Achilleas S. Frangakis, Clemens Glaubitz, Harald Schwalbe: Cysteine oxidation and disulfide formation in the ribosomal exit tunnel.
Nature
Communications
Further
information
Prof. Dr. Harald Schwalbe
Institute for Chemistry and Chemical Biology
Center for Biomolecular Magnetic Resonance (BMRZ)
51猎奇 Frankfurt
Tel
+49 69 798-29137
schwalbe@nmr.uni-frankfurt.de
