Small angle neutron scattering and lipidomic analysis of a native, trimeric PSI-SMALP from a thermophilic cyanobacteria

Alternatives to Styrene- and Diisobutylene-Based Copolymers for Membrane Protein Solubilization via Nanodisc Formation

Styrene-maleic acid copolymers (SMAs), and related amphiphilic copolymers, are promising tools for isolating and studying integral membrane proteins in a native-like state. However, they do not exhibit this ability universally, as several reports have found that SMAs and related amphiphilic copolymers show little to no efficiency when extracting specific membrane proteins. Recently, it was discovered that esterified SMAs could enhance the selective extraction of trimeric Photosystem I from the thylakoid membranes of thermophilic cyanobacteria; however, these polymers are susceptible to saponification that can result from harsh preparation or storage conditions. To address this concern, we herein describe the development of α-olefin-maleic acid copolymers (αMAs) that can extract trimeric PSI from cyanobacterial membranes with the highest extraction efficiencies observed when using any amphiphilic copolymers, including diisobutylene-co-maleic acid (DIBMA) and functionalized SMA samples. Furthermore, we will show that αMAs facilitate the formation of photosystem I-containing nanodiscs that retain an annulus of native lipids and a native-like activity. We also highlight how αMAs provide an agile, tailorable synthetic platform that enables fine-tuning hydrophobicity, controllable molar mass, and consistent monomer incorporation while overcoming shortcomings of prior amphiphilic copolymers.

Effects of Esterified Styrene-Maleic Acid Copolymer Degradation on Integral Membrane Protein Extraction

The detergent-free extraction of integral membrane proteins using styrene-maleic acid copolymers (SMAs) has shown promise as a potentially effective technique to isolate proteins in a more native-like conformation. As the field continues to develop, the protein selectivity and extraction efficiency of many analogues of traditional SMAs are being investigated. Recently, we discovered that the monoesterification of SMAs with alkoxy ethoxylate sidechains drastically affects the bioactivity of these copolymers in the extraction of photosystem I from the cyanobacterium Thermosynechococcus elongatus. However, subsequent investigations also revealed that the conditions under which these esterified SMA polymer analogues are prepared, purified, and stored can alter the structure of the alkoxy ethoxylate-functionalized SMA and perturb the protein extraction process. Herein, we demonstrate that the basic conditions required to solubilize SMA analogues may lead to deleterious saponification side reactions, cleaving the sidechains of an esterified SMA and dramatically decreasing its efficacy for protein extraction. We found that this process is highly dependent on temperature, with polymer samples being prepared and stored at lower temperatures exhibiting significantly fewer saponification side reactions. Furthermore, the effects of small-molecule impurities and exposure to light were also investigated, both of which are shown to have significant effects on the polymer structure and/or protein extraction process.