Fragments from alpha-actinin insert into reconstituted lipid bilayers

Neisseria meningitidis Opc invasin binds to the cytoskeletal protein alpha-actinin

Neisseria meningitidis Opc protein is an effective invasin for human endothelial cells. We have investigated novel human endothelial receptors targeted by Opc and observed that Opc-expressing bacteria interacted with a 100 kDa protein in whole-cell lysates of human endothelial and epithelial cells. The identity of the protein was established as alpha-actinin by mass spectrometry. Opc expression was essential for the recognition of alpha-actinin whether provided in a purified form or in cell extracts. The interaction of the two proteins did not involve intermediate molecules. As there was no demonstrable expression of alpha-actinin on the surfaces of any of the eight cell lines studied, the likelihood of the interactions after meningococcal internalization was examined. Confocal imaging demonstrated considerable colocalization of N. meningitidis with alpha-actinin especially after a prolonged period of internalization. This may imply that bacteria and alpha-actinin initially occur in separate compartments and co-compartmentalization occurs progressively over the 8 h infection period used. In conclusion, these studies have identified a novel and an intracellular target for the N. meningitidis Opc invasin. Since alpha-actinin is a modulator of a variety of signalling pathways and of cytoskeletal functions, its targeting by Opc may enable bacteria to survive/translocate across endothelial barriers.

Protein-lipid interactions: correlation of a predictive algorithm for lipid-binding sites with three-dimensional structural data

Background

Over the past decade our laboratory has focused on understanding how soluble cytoskeleton-associated proteins interact with membranes and other lipid aggregates. Many protein domains mediating specific cell membrane interactions appear by fluorescence microscopy and other precision techniques to be partially inserted into the lipid bilayer. It is unclear whether these protein-lipid-interactions are dependent on shared protein motifs or unique regional physiochemistry, or are due to more global characteristics of the protein.

Results

We have developed a novel computational program that predicts a protein's lipid-binding site(s) from primary sequence data. Hydrophobic labeling, Fourier transform infrared spectroscopy (FTIR), film balance, T-jump, CD spectroscopy and calorimetry experiments confirm that the interfaces predicted for several key cytoskeletal proteins (alpha-actinin, Arp2, CapZ, talin and vinculin) partially insert into lipid aggregates. The validity of these predictions is supported by an analysis of the available three-dimensional structural data. The lipid interfaces predicted by our algorithm generally contain energetically favorable secondary structures (e.g., an amphipathic alpha-helix flanked by a flexible hinge or loop region), are solvent-exposed in the intact protein, and possess favorable local or global electrostatic properties.

Conclusion

At present, there are few reliable methods to determine the region of a protein that mediates biologically important interactions with lipids or lipid aggregates. Our matrix-based algorithm predicts lipid interaction sites that are consistent with the available biochemical and structural data. To determine whether these sites are indeed correctly identified, and whether use of the algorithm can be safely extended to other classes of proteins, will require further mapping of these sites, including genetic manipulation and/or targeted crystallography.

Actin and amphiphilic polymers influence on channel formation by Syringomycin E in lipid bilayers

The bacterial lipodepsipeptide syringomycin E (SRE) added to one (cis-) side of bilayer lipid membrane forms voltage dependent ion channels. It was found that G-actin increased the SRE-induced membrane conductance due to formation of additional SRE-channels only in the case when actin and SRE were applied to opposite sides of a lipid bilayer. The time course of conductance relaxation depended on the sequence of SRE and actin addition, suggesting that actin binds to the lipid bilayer and binding is a limiting step for SRE-channel formation. G-actin adsorption on the membrane was irreversible. The amphiphilic polymers, Konig's polyanion (KP) and poly(Lys, Trp) (PLT) produced the actin-like effect. It was shown that the increase in the SRE membrane activity was due to hydrophobic interactions between the adsorbing molecules and membrane. Nevertheless, hydrophobic interactions were not sufficient for the increase of SRE channel-forming activity. The dependence of the number of SRE-channels on the concentration of adsorbing species gave an S-shaped curve indicating cooperative adsorption of the species. Kinetic analysis of SRE-channel number growth led to the conclusion that the actin, KP, and PLT molecules form aggregates (domains) on the trans-monolayer. It is suggested that an excess of SRE-channel formation occurs within the regions of the cis-monolayer adjacent to the domains of the adsorbed molecules, which increase the effective concentration of SRE-channel precursors.

Phosphoinositide binding inhibits alpha-actinin bundling activity

alpha-Actinin is an abundant actin-bundling and adhesion protein that directly links actin filaments to integrin receptors. Previously, in platelet-derived growth factor-treated fibroblasts, we demonstrated that phosphoinositides bind to alpha-actinin, regulating its localization (Greenwood, J. A., Theibert, A. B., Prestwich, G. D., and Murphy-Ullrich, J. E. (2000) J. Cell Biol. 150, 627- 642). In this study, phosphoinositide binding and regulation of alpha-actinin function is further characterized. Phosphoinositide binding specificity, determined using a protein-lipid overlay procedure, suggests that alpha-actinin interacts with phosphates on the 4th and 5th position of the inositol head group. Binding assays and mutational analyses demonstrate that phosphoinositides bind to the calponin homology domain 2 of alpha-actinin. Phosphoinositide binding inhibited the bundling activity of alpha-actinin by blocking the interaction of the actin-binding domain with actin filaments. Consistent with these results, excessive bundling of actin filaments was observed in fibroblasts expressing an alpha-actinin mutant with decreased phosphoinositide affinity. We conclude that the interaction of alpha-actinin with phosphoinositides regulates actin stress fibers in the cell by controlling the extent to which microfilaments are bundled.

Alteration in the Temperature-Dependent Content Release Property of Thermosensitive Liposomes in Plasma

The effect of plasma components on the temperature-dependent content release property of thermosensitive liposomes has been described. Temperature-sensitive liposomes containing mitomycin C (MMC) were prepared from dipalmitoylphosphatidylcholine (DPPC liposomes) and a 7 : 3 mixture of DPPC and dipalmitoylophosphatidylglycerol (DPPC/DPPG liposomes). We defined in this study the difference in the content release between 38 degrees C and 44 degrees C as an index of the temperature-dependent content release efficiency (Delta% release). In the absence of rat plasma, the Delta% release of the DPPC liposomes and the DPPC/DPPG liposomes was 83% and 71%, respectively. However, when the release study was conducted with rat plasma, the Delta% release increased to about 96% for both liposomes. In addition, while the DPPC liposomes were destabilized by rat plasma below the gel-to-liquid crystalline phase transition temperature (T(m)), MMC leakage from the DPPC/DPPG liposomes below T(m) was suppressed by rat plasma. Moreover, the plasma protein binding onto lipid bilayer was concomitant with the gel-to-liquid crystalline phase transition and then enhanced the temperature-dependent release from the DPPC/DPPG liposomes. The possible mechanism of interaction between liposomes and plasma proteins, especially serum albumin, was discussed based on differential scanning calorimetry and protein binding experiments.