Abstract
Bipolar tetraether lipids (BTL) have been long thought to play a critical role in allowing thermoacidophiles to thrive under extreme conditions. In the present study, we demonstrated that not all BTLs from the thermoacidophilic archaeon Sulfolobus acidocaldarius exhibit the same membrane behaviors. We found that free-standing planar membranes (i.e., black lipid membranes, BLM) made of the polar lipid fraction E (PLFE) isolated from S. acidocaldarius formed over a pinhole on a cellulose acetate partition in a dual-chamber Teflon device exhibited remarkable stability showing a virtually constant capacitance (~28 pF) for at least 11 days. PLFE contains exclusively tetraethers. The dominating hydrophobic core of PLFE lipids is glycerol dialky calditol tetraether (GDNT, ~90%), whereas glycerol dialkyl glycerol tetraether (GDGT) is a minor component (~10%). In sharp contrast, BLM made of BTL extracted from microvesicles (Sa-MVs) released from the same cells exhibited a capacitance between 36 and 39 pF lasting for only 8 h before membrane dielectric breakdown. Lipids in Sa-MVs are also exclusively tetraethers; however, the dominating lipid species in Sa-MVs is GDGT (>99%), not GDNT. The remarkable stability of BLMPLFE can be attributed to strong PLFE–PLFE and PLFE–substrate interactions. In addition, we compare voltage-dependent channel activity of calcium-gated potassium channels (MthK) in BLMPLFE to values recorded in BLMSa-MV. MthK is an ion channel isolated from a methanogenic that has been extensively characterized in diester lipid membranes and has been used as a model for calcium-gated potassium channels. We found that MthK can insert into BLMPLFE and exhibit channel activity, but not in BLMSa-MV. Additionally, the opening/closing of the MthK in BLMPLFE is detectable at calcium concentrations as low as 0.1 mM; conversely, in diester lipid membranes at such a low calcium concentration, no MthK channel activity is detectable. The differential effect of membrane stability and MthK channel activity between BLMPLFE and BLMSa-MV may be attributed to their lipid structural differences and thus their abilities to interact with the substrate and membrane protein. Since Sa-MVs that bud off from the plasma membrane are exclusively tetraether lipids but do not contain the main tetraether lipid component GDNT of the plasma membrane, domain segregation must occur in S. acidocaldarius. The implication of this study is that lipid domain formation is existent and functionally essential in all kinds of cells, but domain formation may be even more prevalent and pronounced in hyperthermophiles, as strong domain formation with distinct membrane behaviors is necessary to counteract randomization due to high growth temperatures while BTL in general make archaea cell membranes stable in high temperature and low pH environments whereas different BTL domains play different functional roles.
Highlights
Bipolar tetraether lipids (BTL) have been long thought to play a pivotal role in allowing thermoacidophiles to thrive under extreme conditions such as high temperatures and low pH [1,2]
Stability of BLMPLFE Compared to Other Black lipid membranes (BLM)
Being able to retain a constant membrane capacitance for such a long time (≥11 days) is unprecedented for BLM made of membrane capacitance for such a long time (≥11 days) is unprecedented for BLM made of BTL (BLMBTL) or any other naturally occurring lipids, and is indicative of its remarkable
Summary
Bipolar tetraether lipids (BTL) have been long thought to play a pivotal role in allowing thermoacidophiles to thrive under extreme conditions such as high temperatures and low pH [1,2]. Instability of the BLM made of conventional diester lipids has been a major drawback in these applications [5] To remedy this problem, tough materials such as archaea tetraether lipids (reviewed in [6]), triblock copolymers [7,8] and polymerized lipids [3] have been used. Lipid polymerization reduces membrane fluidity, and the thickness of triblock copolymer membranes (typically 10–11 nm [10,11]) is much larger than that of eukaryotic cellular membranes (3.5–4.3 nm [12]) Both membrane thickness and fluidity can affect membrane protein behaviors [13,14]. Biochemical activities can be detected when proteins insert into triblock copolymer or polymerized lipid membranes (reviewed in [15]), it is not clear to what extent the detected activities resemble those seen in membranes made of naturally occurring lipids
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