Janus-Like Nature Leads to Unexpected Protein Oligomer Formation

Engineered proteins can be used as therapeutics, e.g., for diseases such as cancer. The the immune mobilizing monoclonal T-cell receptor (TCR) against cancer (ImmTAC), for example, can target cancer cells and use the patient’s immune response to fight them. While “natural” proteins that require it for their function have evolved to form a stable “solution” under physiological conditions, the solution stability of engineered proteins is an interesting question, and designing proteins to either avoid or promote aggregation or self-assembly could be useful for different applications. Understanding weak protein-protein interactions is important in this context.

Jennifer J. McManus, University of Bristol, UK, and colleagues have investigated weak self-interactions of ImmTAC proteins and discovered an unexpected formation of noncovalent oligomers. The team used computational methods to predict the likely structure of an ImmTAC molecule targeting a particular cancer antigen (ImmTAC1) and to calculate electrostatic surface charges on the protein. They found that the protein has a “Janus-like”, two-faced charge distribution, with a negative charge at one “end” and a positive one at the other.

The protein has a net negative charge overall at the relevant pH value, which could lead one to think repulsion would dominate. However, the anisotropic charge contribution leads to weak attractive forces between two molecules of the protein, resulting in the formation of oligomers. This was confirmed by light scattering experiments and also observed in another protein variant (ImmTAC2). This protein has a larger difference in surface charges and shows more pronounced self-assembly behavior than ImmTAC1.

According to the researchers, the work could be useful for the rational design of proteins regarding self-assembly properties caused by anisotropic surface charges. The insights gained could also be important for the formulation of protein therapeutics.