Lipid polymorphism of the marginal region of plant thylakoid membranes

Abstract

Thylakoid membranes (TMs) of vascular plants exhibit a highly organized architecture comprising two main distinct functional domains which accommodate virtually all components of the photosynthetic machinery responsible for the light reactions: the stacked, appressed granum membranes and the unstacked stroma lamellae. A domain with distinct structural, compositional, and functional characteristics, the marginal region of grana (grana margin, GM) has also been identified, which includes the highly curved region at the periphery of the granum vesicles that is enriched in CURT1 proteins. It is also well established that plant TMs, in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (HII) phase. The I phases were shown to participate in membrane fusion and to be associated with the activity of the photoprotective water-soluble enzyme, the violaxanthin de-epoxidase (VDE); the HIIphase, for its trypsin sensitivity, was proposed to emerge from stroma-exposed lipid-protein assemblies. Considering the location of CURT1 in GM and that it possesses trypsin-sensitivity, it was hypothesized to participate in the formation of the HIIphase. The aim of the present study was to test this hypothesis – via fingerprinting the lipid phase behavior of GM particles isolated from spinach leaves. Our31P-NMR spectroscopy reveals that the GM fraction of TMs does not contain HIIphase signal; it exhibits an isotropic signature. This non-bilayer, isotropic lipid structure, together with CURT1 proteins, thus appear to promote the formation of the highly curved region of TMs.