Role of cardiolipin in stability of integral membrane proteins

Cardiolipin (CL) is a unique phospholipid with a dimeric structure having four acyl chains and two phosphate groups found almost exclusively in certain membranes of bacteria and of mitochondria of eukaryotes. CL interacts with numerous proteins and has been implicated in function and stabilization of several integral membrane proteins (IMPs). While both functional and stabilization roles of CL in IMPs has been generally acknowledged, there are, in fact, only limited number of quantitative analysis that support this function of CL. This is likely caused by relatively complex determination of parameters characterizing stability of IMPs and particularly intricate assessment of role of specific phospholipids such as CL in IMPs stability. This review aims to summarize quantitative findings regarding stabilization role of CL in IMPs reported up to now.

Cardiolipin binding in bacterial respiratory complexes: structural and functional implications

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Structure of Photosystems I and II

Photosynthesis is the major process that converts solar energy into chemical energy on Earth. Two and a half billion years ago, the ancestors of cyanobacteria were able to use water as electron source for the photosynthetic process, thereby evolving oxygen and changing the atmosphere of our planet Earth. Two large membrane protein complexes, Photosystems I and II, catalyze the primary step in this energy conversion, the light-induced charge separation across the photosynthetic membrane. This chapter describes and compares the structure of two Photosystems and discusses their function in respect to the mechanism of light harvesting, electron transfer and water splitting.

Compensatory role of thecis-trans-isomerase and cardiolipin synthase in the membrane fluidity ofPseudomonas putidaDOT-T1E

In Gram-negative bacteria, cell membrane fluidity is influenced by phospholipid head group composition and linked fatty acids. Exposure of Pseudomonas putida to stressing agents results in short- and long-term modifications in membrane lipids. The main adaptive change observed in response to organic solvents in the short term is the cis- to trans-isomerization of unsaturated fatty acids in a reaction mediated by cis/trans-isomerase (CTI); whereas in the long term an increase in cardiolipin content takes place. Despite the interest of these genes in the context of stress responses, the transcriptional regulation of the cti and cls genes has not been studied before. The cti and cls (cardiolipin synthase) genes in the solvent-tolerant P. putida DOT-T1E strain form monocistronic units and are expressed from sigma-70 promoters. Expression from the cls promoter is sixfold higher in the stationary phase than in the log phase, and expression of the cls gene is not influenced by solvents. The cti gene is expressed at fairly constant levels in the log and stationary phase, but its level of expression is moderately upregulated in response to toluene. We used fluorescence polarization assays to show that mutants deficient in the cti gene exhibit less rigid membranes than the wild-type strain, whereas mutants with a knockout in the cls gene exhibit increased membrane rigidity. A double cti/cls mutant has similar membrane rigidity as the wild-type strain, which points towards a compensatory effect of the mutations with regard to membrane fluidity. However, the cls and cls/cti mutants were more sensitive to solvents than the wild-type and the cti mutant because of the impaired functioning of efflux drug transporters.