Interaction of cationic lipoplexes with floating bilayers at the solid-liquid interface.

Neutron reflection has been used to study the interaction of cationic lipoplexes with different model membrane systems. The model membranes used are prepared as "floating" phospholipid bilayers deposited at a silicon/water interface and separated from the solid substrate either by an adsorbed phospholipid bilayer, polymer cushions composed of polyethylene glycol lipids, or a lipid monolayer adsorbed onto a chemically grafted hydrocarbon layer. The cationic lipoplexes studied are those formed by the complexation of calf thymus DNA with dimethyl-dioctadecylammonium bromide (DDAB), with either cholesterol or dioleoyl-L-alpha-phosphatidylethanolamine (DOPE) incorporated as "helper" lipid. The cationic lipoplexes are found to destroy three of the four types of (negatively charged) floating bilayers, with the rate of destruction dependent on the nature of the layer separating the floating bilayer from the silicon substrate. The only bilayers to remain intact after exposure to the lipoplexes were those fabricated above the chemically grafted (octadecyl) hydrocarbon layer. This supports the hypothesis that the high negative charge density of the SiO2 layer on the silicon surface may influence, by way of electrostatic interaction with the cationic lipid, the interaction of the lipoplexes with the model bilayer. It is concluded that the floating bilayer supported on a chemically grafted hydrocarbon layer lends itself perfectly to the study of lipoplex-membrane interactions and, with sufficient exposure time, would allow a detailed characterization of the structures formed at the membrane interface during the interaction.

Ice recrystallization inhibition proteins (IRIPs) and freeze tolerance in the cryophilic Antarctic hair grass Deschampsia antarctica E. Desv.

Antarctic hair grass (Deschampsia antarctica E. Desv.), the only grass indigenous to Antarctica, has well-developed freezing tolerance, strongly induced by cold acclimation. Here, we show that in response to low temperatures, D. antarctica expresses potent recrystallization inhibition (RI) activity that, inhibits the growth of small ice crystals into potentially damaging large ones, is proteinaceous and localized to the apoplasm. A gene family from D. antarctica encoding putative homologs of an ice recrystallization inhibition protein (IRIP) has been isolated and characterized. IRIPs are apoplastically targeted proteins with two potential ice-binding motifs: 1-9 leucine-rich repeats (LRRs) and c. 16 'IRIP' repeats. IRIP genes appear to be confined to the grass subfamily Pooideae and their products, exhibit sequence similarity to phytosulphokine receptors and are predicted to adopt conformations with two ice-binding surfaces. D. antarctica IRIP (DaIRIP) transcript levels are greatly enhanced in leaf tissue following cold acclimation. Transgenic Arabidopsis thaliana expressing a DaIRIP has novel RI activity, and purified DaIRIP, when added back to extracts of leaves from non-acclimated D. antarctica, can reconstitute the activity found in acclimated plants. We propose that IRIP-mediated RI activity may contribute to the cryotolerance of D. antarctica, and thus to its unique ability to have colonized Antarctica.