Formation of polymer lipid nanodiscs for membrane protein studies

Abstract

Despite the crucial role that membrane proteins have in the biological field, the study of their structure is still a challenge for the scientific community. An obstacle is the dependency of their functional structure on the surrounding conditions. In the last decade a new platform involving poly(styrene-co-maleic acid) (SMA)copolymers has been developed for the extraction of membrane proteins without using any detergent into discoidal structures, referred to as nanodiscs. Nevertheless, even though progress has been achieved using this method, limitations of this system are the insolubility of the polymer at pH lower than 8 and the complexation of the acid groups in the presence of divalent cations. The polymer is also produced via an uncontrolled free-radical polymerisation route which means it has a polydisperse and random structure, making it di cult to correlate properties of the polymer with the structures of the nanodiscs.

In this project new styrene maleimide copolymers, soluble at pH lower than SMA, were investigated for their potential in forming stable lipid structures. Characterisation of their architectures was carried out using mainly scattering techniques. The stability of these structures were studied in the presence of divalent cations such as CaCl2 and at low pH.

Moreover, SMA copolymers were synthesised with a narrow distribution of molecular weights and well defined architecture using a controlled radical polymerisation method. This demonstrated that the size and stability of the nanodiscs could be tuned directly via control of polymer structure. To achieve supramolecular assembly at a wider pH range, zwitterionic copolymers were also synthesised and their structures characterised.

The mechanism and kinetics of the polymer insertion into the lipid monolayer and the polymer self-assembly around the lipid tails as nanodiscs formed were investigated using several techniques such as fluorescence, small angle neutron scattering and neutron and X-Ray reflectivity. The mechanism of disc formation is complex and appears to depend on the properties of both the polymer and the lipids.

Date of Award	1 Sept 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Karen Edler (Supervisor)