Membrane binding of MARCKS-related protein studied by tryptophan fluorescence spectroscopy

Membrane bound α-synuclein is fully embedded in the lipid bilayer while segments with higher flexibility remain

Cellular pathways involving α-synuclein (αS) seem to be causative for development of Parkinson's disease. Interactions between αS and lipid membranes appear to be important for the physiological function of the protein and influence the pathological aggregation of αS leading to the formation of amyloid plaques. Upon membrane binding the unstructured αS folds into amphipathic helices. In our work we characterized the penetration depth and probed the local environment of Trp-residues introduced along the αS sequence. We could show that while the entire helix is well embedded in the lipid bilayer, segments with a shallower penetration and supposable higher flexibility exist.

MARCKS as a negative regulator of lipopolysaccharide signaling

Myristoylated alanine-rich C kinase substrate (MARCKS) is an intrinsically unfolded protein with a conserved cationic effector domain, which mediates the cross-talk between several signal transduction pathways. Transcription of MARCKS is increased by stimulation with bacterial LPS. We determined that MARCKS and MARCKS-related protein specifically bind to LPS and that the addition of the MARCKS effector peptide inhibited LPS-induced production of TNF-α in mononuclear cells. The LPS binding site within the effector domain of MARCKS was narrowed down to a heptapeptide that binds to LPS in an extended conformation as determined by nuclear magnetic resonance spectroscopy. After LPS stimulation, MARCKS moved from the plasma membrane to FYVE-positive endosomes, where it colocalized with LPS. MARCKS-deficient mouse embryonic fibroblasts (MEFs) responded to LPS with increased IL-6 production compared with the matched wild-type MEFs. Similarly, small interfering RNA knockdown of MARCKS also increased LPS signaling, whereas overexpression of MARCKS inhibited LPS signaling. TLR4 signaling was enhanced by the ablation of MARCKS, which had no effect on stimulation by TLR2, TLR3, and TLR5 agonists. These findings demonstrate that MARCKS contributes to the negative regulation of the cellular response to LPS.

Fluorescence spectroscopy measures yeast PAH1-encoded phosphatidate phosphatase interaction with liposome membranes

Phosphatidate (PA) phosphatase, the enzyme that catalyzes the penultimate step in triacylglycerol synthesis, is a cytosolic enzyme that must associate with the membrane where its substrate PA resides. Fluorescence spectroscopy was used to measure the interaction of yeast PAH1-encoded PA phosphatase with model liposome membranes. PA phosphatase contains five tryptophan residues and exhibited inherit fluorescence that increased upon interaction with phosphatidylcholine liposomes. The interaction was enhanced by inclusion of other phospholipids and especially the substrate PA. Interaction was dependent on both the concentration of phosphatidylcholine-PA liposomes as well as the surface concentration of PA in liposomes. Mg(2+) ions, which were required for catalysis, did not affect PA phosphatase interaction with phosphatidylcholine-PA liposomes. PA phosphatase was a substrate for protein kinase A, protein kinase C, and casein kinase II, and these phosphorylations decreased PA phosphatase interaction with phosphatidylcholine-PA liposome membranes.

Effect of tryptophan oligopeptides on the size distribution of POPC liposomes: a dynamic light scattering and turbidimetric study

A chemical regulation of POPC liposome size distribution was investigated, based on the affinity of indole-containing compounds for phosphocholine membranes. In particular, tryptophan oligopeptides have shown interesting properties of size regulation, both when liposomes were formed in their presence and when the peptides were added to a preformed liposome suspension. Combining dynamic light scattering (DLS) and turbidimetric data, it was possible to show how such peptides had an influence on the size distribution of spontaneously formed liposomes prepared by the thin film hydration, reverse-phase evaporation and ethanol (or methanol) injection methods. In the presence of Trp-Trp or Trp-Trp-Trp, a disappearance of large vesicle aggregates was observed, as suggested also by light microscopy analysis. On the contrary, no effect was detected using extruded vesicles. Turbidimetric titration allowed the determination of the relative efficacy of the size regulators, Trp-Trp-Trp being about 20 times more powerful than the dimer, while the monomer had no effect. In addition, other indole-containing compounds and the antimicrobial peptide indolicidin were tested, showing similar behaviours. Discussing the results according to the current knowledge about the preference of Trp residues for interfacial regions in lecithin bilayers, this study confirms the relevant role of tryptophan in the biomembrane binding properties of many peptides and introduces a new behavior in the field of liposomes-peptides interactions.

Interaction of dystrophin rod domain with membrane phospholipids. Evidence of a close proximity between tryptophan residues and lipids

Dystrophin is assumed to act via the central rod domain as a flexible linker between the amino-terminal actin binding domain and carboxyl-terminal proteins associated with the membrane. The rod domain is made up of 24 spectrin-like repeats and has been shown to modify the physical properties of lipid membranes. The nature of this association still remains unclear. Tryptophan residues tend to cluster at or near to the water-lipid interface of the membrane. To assess dystrophin rod domain-membrane interactions, tryptophan residues properties of two recombinant proteins of the rod domain were examined by (1)H NMR and fluorescence techniques in the presence of membrane lipids. F114 (residues 439-553) is a partly folded protein as inferred from (1)H NMR, tryptophan fluorescence emission intensity, and the excited state lifetime. By contrast, F125 (residues 439-564) is a folded compact protein. Tryptophan fluorescence quenching shows that both proteins are characterized by structural fluctuations with their tryptophan residues only slightly buried from the surface. In the presence of negatively charged small vesicles, the fluorescence characteristics of F125 change dramatically, indicating that tryptophan residues are in a more hydrophobic environment. Interestingly, these modifications are not observed with F114. Fluorescence quenching experiments confirm that tryptophan residues are shielded from the solvent in the complex F125 lipids by a close contact with lipids. The use of membrane-bound quenchers allowed us to conclude that dystrophin rod domain lies along the membrane surface and may be involved in a structural array comprising membrane and cytoskeletal proteins as well as membrane lipids.

Cross-talk unfolded: MARCKS proteins

The proteins of the MARCKS (myristoylated alanine-rich C kinase substrate) family were first identified as prominent substrates of protein kinase C (PKC). Since then, these proteins have been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as endo-, exo- and phago-cytosis, and neurosecretion. The effector domain of MARCKS proteins is phosphorylated by PKC, binds to calmodulin and contributes to membrane binding. This multitude of mutually exclusive interactions allows cross-talk between the signal transduction pathways involving PKC and calmodulin. This review focuses on recent, mostly biophysical and biochemical results renewing interest in this protein family. MARCKS membrane binding is now understood at the molecular level. From a structural point of view, there is a consensus emerging that MARCKS proteins are "natively unfolded". Interestingly, domains similar to the effector domain have been discovered in other proteins. Furthermore, since the effector domain enhances the polymerization of actin in vitro, MARCKS proteins have been proposed to mediate regulation of the actin cytoskeleton. However, the recent observations that MARCKS might serve to sequester phosphatidylinositol 4,5-bisphosphate in the plasma membrane of unstimulated cells suggest an alternative model for the control of the actin cytoskeleton. While myristoylation is classically considered to be a co-translational, irreversible event, new reports on MARCKS proteins suggest a more dynamic picture of this protein modification. Finally, studies with mice lacking MARCKS proteins have investigated the functions of these proteins during embryonic development in the intact organism.