Membrane lipids are both the substrates and a mechanistically responsive environment of TMEM16 scramblase proteins

Abstract

Recent discoveries about functional mechanisms of proteins in the TMEM16 family of phospholipid scramblases have illuminated the dual role of the membrane as both the substrate and a mechanistically responsive environment in the wide range of physiological processes and genetic disorders in which they are implicated. This is highlighted in the review of recent findings from our collaborative investigations of molecular mechanisms of TMEM16 scramblases that emerged from iterative functional, structural, and computational experimentation. In the context of this review, we present new MD simulations and trajectory analyses motivated by the fact that new structural information about the TMEM16 scramblases is emerging from cryo-EM determinations in lipid nanodiscs. Because the functional environment of these proteins in in vivo and in in vitro is closer to flat membranes, we studied comparatively the responses of the membrane to the TMEM16 proteins in flat membranes and nanodiscs. We find that bilayer shapes in the nanodiscs are very different from those observed in the flat membrane systems, but the function-related slanting of the membrane observed at the nhTMEM16 boundary with the protein is similar in the nanodiscs and in the flat bilayers. This changes, however, in the bilayer composed of longer-tail lipids, which is thicker near the phospholipid translocation pathway, which may reflect an enhanced tendency of the long tails to penetrate the pathway and create, as shown previously, a nonconductive environment. These findings support the correspondence between the mechanistic involvement of the lipid environment in the flat membranes, and the nanodiscs.

TMEM16 scramblases thin the membrane to enable lipid scrambling

“…To test whether the long C22 acyl chains inhibit scrambling in saturating Ca 2+ by occluding the pathway 40 we measured scrambling in membranes formed by 70% C22 lipids and 30% C18 lipids, which are ~4 nm thick (Table 1 ). In saturating Ca 2+ scrambling activity is similar to that seen in 100% C18 lipids (Fig.…”

Confinement in Nanodiscs Anisotropically Modifies Lipid Bilayer Elastic Properties

“… 9 This technology has furthermore afforded key insights into the mechanistic role of the lipid environment in modulating the functional and structural properties of membrane proteins. 10 …”

“…Indeed, the direct relation that was found between the lipid environment and the functional efficiency of the TMEM16 scramblase was attributed to the strongly slanted shape of the membrane around the protein. 10 , 23 − 25 Other examples include various transporter and channel proteins for which the membrane regulation mechanism has been related to contributions from bilayer deformation energy, curvature frustration, and lipid packing stress, among other factors (see refs ( 26 ) and ( 27 ) and citations therein). Thus, a quantitative description of the elastic properties of nanodisc membranes is potentially crucial for understanding how proteins embedded in nanodiscs are structurally and functionally influenced by the surrounding lipid environment.…”

TMEM16 scramblases thin the membrane to enable lipid scrambling

“…4). To test whether the long C22 acyl chains inhibit scrambling in saturating Ca 2+ by occluding the pathway 37 we measured scrambling in membranes formed by 70% C22 lipids and 30% C18 lipids, which are ~4 nm thick (Table 1).…”