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3 Dec 2013

Self-Assembling Cages from Coiled-Coil Peptide Modules

This paper in Science caught my eye earlier in the year. It is another peptide biomaterials paper for which I make no apology – this seems to be a really exciting area of peptide science right now.

This is a publication from the laboratory of Prof. Dek Woolfson, University of Bristol, whose work is largely based around α-helical coiled-coil structures. They are using bioinformatics to assemble a relational database of coiled-coil structures from the Protein Data Bank (PDB) which they use to probe understanding of the sequence-to-structure relationships in coiled-coil protein structures. They have used this understanding to publish a ‘Periodic Table of Coiled-coil Structures’ and use these tools to develop rules for coiled-coil folding and assembly to guide de novo coiled-coil designs.

The lab then uses synthetic biology approaches to build a series of peptide toolkits that can be used to assemble higher-order structures such as homo-dimers, -trimers and –tetramers and a number of hetero-dimers. This work has been published in ACS Synthetic Biology. These are synthetic peptides around 30 residues in length that assemble reversibly and form stable structures at µM-nM concentrations.

This work has now been extended to make self-assembled cage-like particles (‘SAGEs’). These are made using a shorter (approx. 20 residues) homo-trimer coiled-coils that they call CC-Tri3 that form the core of a ‘hub’.

 

 

CC-Tri3

Additionally they prepared coiled-coil hetero-dimers, one consisting of one acidic coiled coil (CC-Di-A, red in the diagram) and one basic coiled coil (CC-Di-B, blue in the diagram) so they are held together by salt bridges.

CC-DiA-DiB

If either an acidic (CC-Di-A)  or a basic (CC-Di-B) coiled coil is linked through disulfide bonds to the homo-trimer (CC-Tri3), then two types of hub can be formed, hub A (left) and hub B (right):

 

Hub_Formation

These hubs can then be mixed and will associate via salt-bridges between the acidic and basic coiled-coils. This should produce a hexagonal network with a pore size of about 5 – 6 nm.

 

HexagonalNetwork

 

However, rather than producing flat sheets, this structure folds on itself to maximise the coiled-coil interactions and thereby produces closed objects that the authors call SAGE’s.

This would be impressive enough, but the Woolfson group has gone further. By making a subtle change to the amino acid sequence (Asn à Ile) at the interface between the two strands of the hetero-dimer, it is possible to change the structure slightly. This slight change has quite a significant impact on the nanostructure, reducing the diameter of the CAGEs produced from 97nm to 68nm. By using other modifications, structures in the range 50 – 150nm can be produced.

Because the group has so much experience in manipulating the interactions between these coiled-coil structures, it is possible for them to use this as a test system to investigate production of new nanostructures.

These CAGEs offer many exciting possibilities. An obvious one is encapsulation, either by simple entrapment or by incorporation of specific functionality on the inside surface of the CAGE. The Woolfson group is believed to be developing this application in collaboration with at least one industrial partner. Other possibilities include modifying the surface of the particles to facilitate cell binding, antigen binding (e.g. vaccine delivery or cell targeting).

The work has already generated significant interest and has been review twice, once in the same issue of Science as the original publication – if you have an online subscription you can read it here. More recently, it was reviewed in Chemical & Engineering News which you can read if you have an ACS subscription.

 

Stephen Hoare, Owner / Principal at Peptide Conferences