Effects of phospholipases C and D on ordering of channel proteins in the mitochondrial outer membrane

Phospholipase D in cell signalling and its relationship to phospholipase C

Phospholipases C and D are phosphodiesterases which act on phospholipid head groups. Although the presence of these enzymes in living organisms has long been known, it is only recently that their role in cell signal transduction has been appreciated. The new developments on phospholipases D (PLD) are especially noteworthy, since these enzymes catalyze a novel pathway for second messenger generation. In a variety of mammalian cell systems, several biological or chemical agents have recently been shown to stimulate PLD activity. Depending on the system, activation of PLD has been suggested to be either dependent on, or independent of, Ca2+ and protein kinase C. PLD primarily hydrolyses phosphatidylcholine (PC) but phosphatidylinositol and phosphatidylethanolamine have also been reported as substrates. Different forms of endogenous PLD may also exist in cells. Exogenous addition of PLD causes alterations in cellular functions. In many instances, Ca2+ mobilizing agonists may stimulate both PLC and PLD pathways. Interestingly, several metabolites of these two enzymes are second messengers and are common to both pathways (e.g. phosphatidic acid, diglyceride). This has raised the issue of the interrelationship between these pathways. The regulation of either PLC or PLD by cellular components, e.g. guanine nucleotide binding proteins or protein kinases, is under intense investigation. These recent advances are providing novel information on the significance of phospholipase C and D mediated phospholipid turnover in cellular signalling. This review highlights some of these new discoveries and emerging issues, as well as challenges for future research on phospholipases.

Lipid topology and physical properties of the outer mitochondrial membrane of the yeast, Saccharomyces cerevisiae

The outer membrane of yeast mitochondria was studied with respect to its lipid composition, phospholipid topology and membrane fluidity. This membrane is characterized by a high phospholipid to protein ratio (1.20). Like other yeast cellular membranes the outer mitochondrial membrane contains predominantly phosphatidylcholine (44% of total phospholipids), phosphatidylethanolamine (34%) and phosphatidylinositol (14%). Cardiolipin, the characteristic phospholipid of the inner mitochondrial membrane (13% of total phospholipids) is present in the outer membrane only to a moderate extent (5%). The ergosterol to phospholipid ratio is higher in the inner (7.0 wt%) as compared to the outer membrane (2.1 wt.%). Attempts to study phospholipid asymmetry by selective degradation of phospholipids of the outer leaflet of the outer mitochondrial membrane failed, because isolated right-side-out vesicles of this membrane became leaky upon treatment with phospholipases. Selective removal of phospholipids of the outer leaflet with the aid of phospholipid transfer proteins and chemical modification with trinitrobenzenesulfonic acid on the other hand, gave satisfactory results. Phosphatidylcholine and phosphatidylinositol are more or less evenly distributed between the two sides of the outer mitochondrial membrane, whereas the majority of phosphatidylethanolamine is oriented towards the intermembrane space. The fluidity of mitochondrial membranes was determined by measuring fluorescence anisotropy using diphenylhexatriene (DPH) as a probe. The lower anisotropy of DPH in the outer as compared to the inner membrane, which is an indication for an increased lipid mobility in the outer membrane, was attributed to the higher phospholipid to protein and the lower ergosterol to phospholipid ratio. The data presented here show, that the outer mitochondrial membrane, in spite of its close contact to the inner membrane, is distinct not only with respect to its protein pattern, but also with respect to its lipid composition and physical membrane properties.

Electron microscopy and image analysis of the mitochondrial outer membrane channel, VDAC

The channel protein in the outer membrane of Neurospora crassa mitochondria, VDAC, forms extended planar crystals on the membrane. The arrays, which are induced by phospholipase A2, are polymorphic, varying from parallelogram (P) to near-rectangular (R) geometry with increased phospholipase treatment. Computer-based analysis of projection images of negatively stained VDAC arrays indicates that the protein forms a transmembrane channel in the P array. Comparison of average images of arrays embedded in different negative stains suggests that the bore of the channel is 2-2.5 nm. The locations of functionally important lysine clusters on VDAC are inferred from the effects of succinylation on projection images of arrays negatively stained with phosphotungstate. Projection images of unstained frozen-hydrated arrays indicate the general shape of the channel and suggest each channel is formed by one 31-kDa VDAC polypeptide.