Adhesion between liposomes mediated by the chlorophyll a/b light-harvesting complex isolated from chloroplast membranes

Abstract

A highly purified chlorophyll a/b light-harvesting complex (chi a/b LHC; chi a/b ratio 1.2) was obtained from Triton-solubilized chloroplast membranes of pea and barley according to the method of Burke et al. (1978, Arch. Biochem. Biophys. 187: 252-263). Gel electrophoresis of the cation-precipitated chi a/b LHC from peas reveals the presence of four polypeptides in the 23- to 28-kdalton size range. Three of these peptides appear to be identical to those derived from re-electrophoresed CPII and CPII* bands. In freeze-fracture replicas, the cation-precipitated chi a/b LHC appears as a semicrystalline aggregate of membranous sheets containing closely spaced granules. Upon removal of the cations by dialysis, the aggregates break up into their constituent membranous sheets without changing their granular substructure. These membranous sheets can be resolubilized in 1.5 Triton X-100, and the chi a/b LHC particles then reconstituted into soybean lecithin liposomes. Freeze-fracture micrographs of the reconstituted chi a/b LHC vesicles suspended in a low salt medium reveal randomly dispersed ~80-Å particles on both concave and convex fracture faces as well as some crystalline particle arrays, presumably resulting from incompletely solubilized fragments of the membranous sheets. Based on the ~80-Å diameter of the particles, and on the assumption that one freeze-fracture particle represents the structural unit of one chi a/b LHC aggregate, a theoretical mol wt of ~200 kdalton has been calculated for the chi a/b LHC. Deep-etching and negative-staining techniques reveal that the chi a/b LHC particles are also exposed on the surface of the bilayer membranes. Addition of ≥2 mM MgCL or ≥60 mM NaCl to the reconstituted vesicles leads to their aggregation and, with divalent cations, to the formation of extensive membrane stacks. At the same time, the chi a/b LHC particles become clustered into the adhering membrane regions. Under these conditions the particles in adjacent membranes usually become precisely aligned. Evidence is presented to support the hypothesis that adhesion between the chi a/b LHC particles is mediated by hydrophobic interactions, and that the cations are needed to neutralize surface charges on the particles. © 1980, Rockefeller University Press., All rights reserved.