Fragments from actin binding protein (ABP-280; filamin) insert into reconstituted lipid layers

New user-friendly approach to obtain an Eisenberg plot and its use as a practical tool in protein sequence analysis

The Eisenberg plot or hydrophobic moment plot methodology is one of the most frequently used methods of bioinformatics. Bioinformatics is more and more recognized as a helpful tool in Life Sciences in general, and recent developments in approaches recognizing lipid binding regions in proteins are promising in this respect. In this study a bioinformatics approach specialized in identifying lipid binding helical regions in proteins was used to obtain an Eisenberg plot. The validity of the Heliquest generated hydrophobic moment plot was checked and exemplified. This study indicates that the Eisenberg plot methodology can be transferred to another hydrophobicity scale and renders a user-friendly approach which can be utilized in routine checks in protein–lipid interaction and in protein and peptide lipid binding characterization studies. A combined approach seems to be advantageous and results in a powerful tool in the search of helical lipid-binding regions in proteins and peptides. The strength and limitations of the Eisenberg plot approach itself are discussed as well. The presented approach not only leads to a better understanding of the nature of the protein–lipid interactions but also provides a user-friendly tool for the search of lipid-binding regions in proteins and peptides.

Novel structural and functional findings of the ehFLN protein from Entamoeba histolytica

The ehFLN protein (previously known as EhABP-120) is the first filamin to be identified in the parasitic protozoan Entamoeba histolytica. Filamins are a family of cross-linking actin-binding proteins that organize filamentous actin in networks and stress fibers. It has been reported that filamins of different organisms directly interact with more than 30 cellular proteins and some PPIs. The biochemical consequences of such interactions may have either positive or negative effects on the cross-linking function. Besides, filamins form a link between cytoskeleton and plasma membrane. In this work, the ehFLN protein was biochemically characterized; amoebae filamin was found to associate with both PA and PI(3)P in vitro, new lipid targets for a member of the filamins. By molecular modeling analysis and protein-lipid overlay assays, K-609, 709, and 710 were determined to be essential for the PA-ehFLN1 complex stability. Also, the integrity of the 4th repeat of ehFLN is essential to keep interaction with the PI(3)P. Transfected trophozoites that overexpressed the d100, d50NH(2), and d50COOH regions of ehFLN1 displayed both increased motility and chemotactic response to TYI-S-33 media. Together, these results suggest that short regions of ehFLN are involved in signaling events that, in cooperation with phosphatidic acid, EhPLD2 and EhPI3K, could promote cell motility.

Actin-binding protein filamin A is displayed on the surface of human neuroblastoma cells

We previously reported the identification of natural human IgM antibodies, which recognize a M(r) 260 000 surface protein (NB-p260) and induce both complement-mediated cytotoxicity and apoptosis of human neuroblastoma cells. NB-p260 was shown to belong to the family of filamin proteins. Filamin A is a high molecular weight actin-binding protein, previously thought to be only located intracellularly. Here we show that NB cells as well as three NB-unrelated human cell lines express filamin A also on the cell surface. Our findings suggest new biological functions for filamins, including a role as mediators in anti-NB IgM-induced apoptosis, and they add to the growing body of evidence of the interaction of cytoskeletal proteins with the extracellular matrix.

Filamin A is required for T cell activation mediated by protein kinase C-theta

Induction of T cell responses following engagement of the Ag-specific TCR depends on TCR-initiated rearrangements of the cellular actin cytoskeleton and highly coordinated and tightly regulated interactions and of diverse intracellular signaling proteins. In this study, we show that filamin A (FLNa), an actin-binding and signal mediator scaffolding protein, is required for T cell activation. Following Ag stimulation, FLNa was recruited to the T cell-APC contact area, where it colocalized with protein kinase C-theta (PKCtheta). Depletion of FLNa by RNA interference did not affect TCR-induced early tyrosine phosphorylation or actin polymerization but, nevertheless, resulted in impaired IL-2 expression by human primary T cells and reduced activation of NF-kappaB, AP-1, and NFAT reporter genes in transfected T cells. TCR stimulation induced stable physical association of FLNa with PKCtheta. Furthermore, the TCR/CD28-induced membrane translocation of PKCtheta was inhibited in FLNa-depleted T cells. These results reveal novel role for FLNa in the TCR/CD28 signaling pathway leading to transcription factor activation and IL-2 production, and suggest that this role is mediated, in part, through the inducible interaction of FLNa with PKCtheta.

The ABP-120 C-end region from Entamoeba histolytica interacts with sulfatide, a new lipid target

EhABP-120 is the first filamin identified in the parasitic protozoan Entamoeba histolytica. Filamins are a family of cross-linking actin-binding proteins that promote a dynamic orthogonal web. They have been reported to interact directly with more than 30 cellular proteins and some phosphoinositides. The biochemical consequences of these interactions may have either positive or negative effects on the cross-linking function and also form a link between the cytoskeleton and plasma membrane. In this study, the EhABP-120 carboxy-terminal domain (END) was biochemically characterized. This domain was able to associate to 3-sulfate galactosyl ceramide, a new lipid target for a member of the filamin family. Also, the END domain was able to dimerize "in vitro." Molecular modeling analysis showed that the dimeric region is stabilized by a disulfide bond. Electrostatic and docking studies suggest that an electropositive concave pocket at the dimeric END domain interacts simultaneously with several sulfogalactose moieties of the sulfatide.

Viral membrane penetration: lytic activity of a nodaviral fusion peptide

The auto-cleavage product from the C-terminal part of the capsid protein of the flock house virus, namely the gamma(1) peptide, was used as a model peptide to characterize the initial steps of viral membrane penetration. Monolayers at the air-water interface were used to investigate the phase behaviour of ternary lipid-peptide mixtures, whereas solid-supported membranes were used to visualize the lytic activity of the gamma(1) peptide. 1,2-Dipalmitoyl-sn-glycero-phospatidylcholine/1,2-dipalmitoyl-sn-glycero-phospatidylserine (4:1) membranes were used as negatively charged model membranes. By means of film balance techniques lipid/peptide discrimination was found resulting in a lipid-rich and a peptide-rich phase. Quartz crystal microbalance and scanning force microscopy experiments led to the conclusion of a detergent-like mechanism of the gamma(1) peptide resulting in mixed lipid-peptide micelles with a molar ratio of 2.8:1. A monolayer adsorption with an ongoing lysis of membranes was found with gamma(1) peptide molecules interacting at membrane defects.

The F-actin cross-linking and focal adhesion protein filamin A is a ligand and in vivo substrate for protein kinase C alpha

Filamin A is an established structural component of cell-matrix adhesion sites. In addition, it serves as a scaffold for the subcellular targeting of different signaling molecules. Protein kinase C (PKC) has been found associated with filamin; however, details about this interaction and its significance for cell-matrix adhesion-dependent signaling have remained elusive. We performed a yeast two-hybrid analysis using protein kinase Calpha as a bait and identified filamin as a direct binding partner. The interaction was confirmed in transfected HeLa cells, and serial truncation fragments of filamin A were employed to identify two binding sites on filamin. In vitro ligand binding assays revealed a Ca2+ and phospholipid-dependent association of the regulatory domain of protein kinase C with these sites. Phosphorylation of filamin was found to be isoform-restricted, leading to phosphate incorporation in the C termini of filamin A and C, but not B. PKC-dependent phosphorylation of filamin was also detected in cells. Our data suggest an intimate interaction between filamin and PKC in cell signaling.

Fragments from alpha-actinin insert into reconstituted lipid bilayers

Recent experiments have indicated that alpha-actinin interacts with phospholipid membranes. Using computer analysis methods we determined two possible lipid binding sites capable of membrane attachment/insertion, residues 281-300 and 720-739 of the primary amino acid sequence on smooth muscle alpha-actinin. Having expressed these regions as fusion proteins with schistosomal GST (glutathione S-transferase), we used differential scanning calorimetry (DSC) to investigate their interaction with mixtures of zwitterionic (dimyristoyl-l-alpha-phosphatidylcholine, DMPC) and anionic (dimyristoyl-l-alpha-phosphatidylglycerol, DMPG) phospholipids in reconstituted lipid bilayers. Calorimetric measurements showed that as fusion protein concentration increased, the main chain transition enthalpy decreased and chain melting temperatures shifted, which is indicative of partial protein insertion into the hydrophobic region of the lipid membranes. Centrifugation assay and subsequent SDS/Page chromatography confirmed this finding.