Membrane organization and ionization behavior of the minor but crucial lipid ceramide-1-phosphate

¹ Kent State University
² Universidad del Pais Vasco
³ Univ. del Pais Vasco
⁴ Univ Utrecht
⁵ Universidad del Pais Vasco Departamento de Bioquimica

^* To whom correspondence should be addressed. E-mail: e.e.kooijman{at}gmail.com.

Submitted on September 4, 2007
Revised on October 9, 2007
Accepted on 14 January 2008

	Abstract

Ceramide-1-phosphate (Cer-1-P), one of the simplest of all sphingophospholipids, occurs in minor amounts in biological membranes. Yet recent evidence suggests important roles of this lipid as a novel second messenger with crucial tasks in cell survival and inflammatory responses. Here we present a detailed description of the physical chemistry of this hitherto little explored membrane lipid. At full hydration Cer-1-P forms a highly organized subgel (crystalline) bilayer phase (L_c) at low temperature, which transforms into a regular gel phase (L) at ~45°C, with the gel to fluid phase transition (L-L) occurring at ~65°C. When incorporated at 5 mol % in a phosphatidylcholine bilayer, the pKa₂ of Cer-1-P, 7.39[±0.03, lies within the physiological pH range. Inclusion of phosphatidylethanolamine in the phosphatidylcholine bilayer, at equimolar ratio, dramatically reduces the pKa₂ to 6.64±0.03. We explain these results in light of the novel electrostatic/hydrogen bond switch model recently described for phosphatidic acid. In mixtures with dielaidoylphosphatidylethanolamine, small concentrations of Cer-1-P cause a large reduction of the lamellar-to-inverted hexagonal phase transition temperature, suggesting that Cer-1-P induces, like PA, negative membrane curvature in these type of lipid mixtures. These properties place Cer-1-P in a class more akin to certain glycerophospholipids (phosphatidylethanolamine, phosphatidic acid) than to any other sphingolipid. In particular, the similarities and differences between ceramide and Cer-1-P may be relevant in explaining some of their physiological roles.

Key Words: DSC, NMR, lipid phase transitions, lipid/membrane electrostatics, lipid/protein interactions, negative curvature, wide angle x-ray diffraction