Styrene maleic acid (SMA) polymers have proven to be very successful for the extraction of membrane proteins, forming SMA lipid particles (SMALPs), which maintain a lipid bilayer around the membrane protein. SMALP-encapsulated membrane proteins can be used for functional and structural studies. The SMALP approach allows retention of important protein-annular lipid interactions, exerts lateral pressure, and offers greater stability than traditional detergent solubilisation. However, SMA polymer does have some limitations, including a sensitivity to divalent cations and low pH, an absorbance spectrum that overlaps with many proteins, and possible restrictions on protein conformational change. Various modified polymers have been developed to try to overcome these challenges, but no clear solution has been found. A series of partially-esterified variants of SMA (SMA 2625, SMA 1440 and SMA 17352) has previously been shown to be highly effective for solubilisation of plant and cyanobacterial thylakoid membranes. It was hypothesised that the partial esterification of maleic acid groups would increase tolerance to divalent cations. Therefore, these partially-esterified polymers were tested for the solubilisation of lipids and membrane proteins, and their tolerance to magnesium ions. It was found that all partially esterified polymers were capable of solubilising and purifying a range of membrane proteins, but the yield of protein was lower with SMA 1440, and the degree of purity was lower for both SMA1440 and SMA17352. SMA2625 performed comparably to SMA 2000. SMA 1440 also showed an increased sensitivity to divalent cations. Thus, it appears the interactions between SMA and divalent cations are more complex than proposed and require further investigation.
© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
Funding: AAG was the recipient of an Aston University overseas student excellence bursary and AA the recipient of a Life and Health Sciences School Studentship. AV is part of the MemTrain programme funded by the European Union's Horizon 2020 - Research and Innovation Framework Programme under H2020 Marie Skłodowska-Curie Actions grant agreement No. 847419 to AJR, RMB and ADG. OPH and SN are part of the MIBTP (Midlands Integrative Biosciences Training Partnership) doctoral training centre funded by the BBSRC (Biotechnology and Biological Sciences Research Council). TRD and AJR received funding from BBSRC (BB/S008160/1).