Helping protein machines in cells form correct 3D structures

20 March 2014

PIH1D1 - a molecular barcode reader for multi-protein assembly.

Image: PIH1D1 - a molecular barcode reader for multi-protein assembly.

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 Define Substrate Specificity of the R2TP Cochaperone Complex, is published in Cell Reports.

  • A collaboration between the Medical Research Council's National Institute for Medical Research and Cancer Research UK's London Research Institute sheds light on a so-called 'protein chaperone' known as R2TP.
  • Chaperone proteins are a diverse family of molecules that help individual proteins or multi-protein assemblies in cells to form the correct, stable three-dimensional arrangement so they can carry out their biological roles.