New research sheds light on the role played by a multi-protein
complex known as R2TP in assembling other multi-protein machines
inside a cell, including some that play crucial roles in
translating genes into proteins.
The study has potential implications for cancer, as one subunit
of R2TP called PIH1D1 is present in abnormally high levels in some
types of cancer cell. This suggests that the growth of some tumours
may also depend on R2TP.
The work was a collaboration by a multidisciplinary team from
the Medical Research Council's National Institute for Medical
Research (NIMR) and Cancer Research UK's London Research Institute
(LRI) (both now part of the Francis Crick Institute).
Dr Steve Smerdon of NIMR explained: "Proteins are the molecular
workhorses of the cell. They are initially produced as linear
chains of amino acids with a precise sequence. However, their
biological activities are ultimately determined by the
three-dimensional shape they adopt.
"Perhaps surprisingly, the means by which a protein folds up
from this linear chain into its final three-dimensional form is
still poorly understood.
"What is known though, is that for many proteins, all of the
information required is contained in the amino acid sequence
itself. However for some, assistance by a family of proteins called
molecular chaperones is necessary for folding to occur
efficiently.
"Many proteins don't exist alone and act in concert with others
as large molecular assemblies. These also require some help to form
correctly and specialised chaperone machineries that are conserved
from yeast to humans, have evolved to achieve this."
R2TP is one such example. Together with other proteins, it has
been shown to play a role in putting together a number of
multi-protein assemblies, such as RNA polymerase, an enzyme that's
crucial for a cell to decode information in its DNA and translate
this into protein.
To find out more about how R2TP acts, the NIMR team used a
technique called X-ray crystallography to visualise the structure
of the PIH1D1 subunit. This revealed how a chemical modification
called phosphorylation acts as a molecular barcode, tagging
proteins that require R2TP's assistance, for recognition by
PIH1D1.
Their colleagues at LRI then used mass spectrometry, which
permits the accurate identification of single proteins from many
thousands in a mixture, to find previously unknown target proteins
for R2TP, including RNA polymerase and regulators of a so-called
'tumour suppressor' protein, p53, that is found to be mutated in
the majority of human cancers.
Dr Smerdon said: "These highly multi-disciplinary studies have
really pushed forward our understanding of the breadth of chaperone
activities in human cells and identified a beautifully specific
mechanism for funneling target proteins into the R2TP machine."
Dr Simon Boulton of LRI added: "Our collaboration with
Steve and his group has been a very enjoyable and productive
endeavour. My hope is that by bringing together world-class
researchers from a range of disciplines, the Crick will foster and
encourage more of this type of collaborative interaction
in the future."
The paper, Phosphorylation-Dependent PIH1D1 Interactions De?ne Substrate
Speci?city of the R2TP Cochaperone Complex, is published inCell Reports.