Adsorption of GST-PI3Kgamma at the air-buffer interface and at substrate and nonsubstrate phospholipid monolayers

Binding of the GTPase Sar1 to a Lipid Membrane Monolayer: Insertion and Orientation Studied by Infrared Reflection⁻Absorption Spectroscopy

Membrane-interacting proteins are polyphilic polymers that engage in dynamic protein–protein and protein–lipid interactions while undergoing changes in conformation, orientation and binding interfaces. Predicting the sites of interactions between such polypeptides and phospholipid membranes is still a challenge. One example is the small eukaryotic GTPase Sar1, which functions in phospholipid bilayer remodeling and vesicle formation as part of the multimeric coat protein complex (COPII). The membrane interaction of Sar1 is strongly dependent on its N-terminal 23 amino acids. By monolayer adsorption experiments and infrared reflection-absorption spectroscopy (IRRAS), we elucidate the role of lipids in inducing the amphipathicity of this N-terminal stretch, which inserts into the monolayer as an amphipathic helix (AH). The AH inserting angle is determined and is consistent with the philicities and spatial distribution of the amino acid monomers. Using an advanced method of IRRAS data evaluation, the orientation of Sar1 with respect to the lipid layer prior to the recruitment of further COPII proteins is determined. The result indicates that only a slight reorientation of the membrane-bound Sar1 is needed to allow coat assembly. The time-course of the IRRAS analysis corroborates a role of slow GTP hydrolysis in Sar1 desorption from the membrane.

Biophysical methods for the characterization of PTEN/lipid bilayer interactions

PTEN, a tumor suppressor protein that dephosphorylates phosphoinositides at the 3-position of the inositol ring, is a cytosolic protein that needs to associate with the plasma membrane or other subcellular membranes to exert its lipid phosphatase function. Upon membrane association PTEN interacts with at least three different lipid entities: An anionic lipid that is present in sufficiently high concentration to create a negative potential that allows PTEN to interact electrostatically with the membrane, phosphatidylinositol-4,5-bisphosphate, which interacts with PTEN's N-terminal end and the substrate, usually phosphatidylinositol-3,4,5-trisphosphate. Many parameters influence PTEN's interaction with the lipid bilayer, for example, the lateral organization of the lipids or the presence of other chemical species like cholesterol or other lipids. To investigate systematically the different steps of PTEN's complex binding mechanism and to explore its dynamic behavior in the membrane bound state, in vitro methods need to be employed that allow for a systematic variation of the experimental conditions. In this review we survey a variety of methods that can be used to assess PTEN lipid binding affinity, the dynamics of its membrane association as well as its dynamic behavior in the membrane bound state.

Negatively charged phospholipids trigger the interaction of a bacterial Tat substrate precursor protein with lipid monolayers

Folded proteins can be translocated across biological membranes via the Tat machinery. It has been shown in vitro that these Tat substrates can interact with membranes prior to translocation. Here we report a monolayer and infrared reflection-absorption spectroscopic (IRRAS) study of the initial states of this membrane interaction, the binding to a lipid monolayer at the air/water interface serving as a model for half of a biological membrane. Using the model Tat substrate HiPIP (high potential iron-sulfur protein) from Allochromatium vinosum, we found that the precursor preferentially interacts with monolayers of negatively charged phospholipids. The signal peptide is essential for the interaction of the precursor protein with the monolayer because the mature HiPIP protein showed no interaction with the lipid monolayer. However, the individual signal peptide interacted differently with the monolayer compared to the complete precursor protein. IRRA spectroscopy indicated that the individual signal peptide forms mainly aggregated β-sheet structures. This β-sheet formation did not occur for the signal peptide when being part of the full length precursor. In this case it adopted an α-helical structure upon membrane insertion. The importance of the signal peptide and the mature domain for the membrane interaction is discussed in terms of current ideas of Tat substrate-membrane interactions.

Improved functional recovery to I/R injury in hearts from lipocalin-2 deficiency mice: restoration of mitochondrial function and phospholipids remodeling

AIMS

Recent clinical and experimental evidences demonstrate an association between augmented circulating lipocalin-2 [a pro-inflammatory adipokine] and cardiac dysfunction. However, little is known about the pathophysi-ological role of lipocalin-2 in heart. The present study was designed to compare the heart functions of mice with normal (WT) or deficient lipocalin-2 (Lcn2-KO) expression.

METHODS AND RESULTS

Echocardiographic analysis revealed that the myocardial contractile function was significantly improved in hearts of Lcn2-KO mice, under both standard chow and high fat diet conditions. The heart function before and after I/R injury (20-min of global ischemia followed by 60-min of reperfusion) was assessed using the Langendorff perfusion system. Compared to WT littermates, hearts from Lcn2-KO mice showed improved functional recovery and reduced infarct size following I/R. Under baseline condition, the mitochondrial function of Lcn2-KO hearts was significantly enhanced, as demonstrated by biochemical analysis of respiratory chain activity and markers of biogenesis, as well as electron microscopic investigation of the mitochondrial ultrastructure. Acute or chronic administration of lipocalin-2 impaired cardiac functional recovery to I/R and dampened the mitochondrial function in hearts of Lcn2-KO mice. These effects were associated with an extensive modification of the fatty acyl chain compositions of intracellular phospholipids. For example, lipocalin-2 facilitated the redistribution of linoleic acid (C18:2) among different types of phospholipids, including cardiolipin, a structurally unique phospholipid located mainly on the inner membrane of mitochondria.

CONCLUSIONS

Lack of lipocalin-2 improved the functional recovery of isolated mice hearts subjected to I/R, which is associated with restoration of mitochondrial function and phospholipids remodeling.