Abstract
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 SMA 1440 and SMA 17352. SMA 2625 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.
Highlights
Membrane proteins are well known to be important targets for furthering our understanding of fundamental biology and the develop ment of novel therapeutics [1]
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
SMA inserts into membranes and forms small discs of lipid bilayer, with the polymer wrapped around the outside, termed SMALPs (SMA lipid particles) [8]
Summary
Membrane proteins are well known to be important targets for furthering our understanding of fundamental biology and the develop ment of novel therapeutics [1]. A series of partially-esterified variants of SMA has previously been tested for the solubilisation of plant and cyanobacterial thylakoid membranes [33,42,43,44] These polymers, SMA 2625, SMA 1440 and SMA 17352, have various chemical groups attached to some of the maleic acid groups of SMA (Fig. 1 & Table 1). These groups have been implicated in the divalent cation sensitivity of SMA, the carboxylates of the maleic acid groups being proposed to chelate Mg2+, inducing strain or a conformational change in the polymer surrounding the SMALP It is thought this process causes the SMA to precipitate [16,22].
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