Choosing the right fluorophore for single-molecule fluorescence studies in a lipid environment

Nonspecific interactions between lipids and fluorophores can alter the outcomes of single-molecule spectroscopy of membrane proteins in live cells, liposomes or lipid nanodiscs and of cytosolic proteins encapsulated in liposomes or tethered to supported lipid bilayers. To gain insight into these effects, we examined interactions between 9 dyes that are commonly used as labels for single-molecule fluorescence (SMF) and 6 standard lipids including cationic, zwitterionic and anionic types. The diffusion coefficients of dyes in the absence and presence of set amounts of lipid vesicles were measured by fluorescence correlation spectroscopy (FCS). The partition coefficients and the free energies of partitioning for different fluorophore-lipid pairs were obtained by global fitting of the titration FCS curves. Lipids with different charges, head groups and degrees of chain saturation were investigated, and interactions with dyes are discussed in terms of hydrophobic, electrostatic and steric contributions. Fluorescence imaging of individual fluorophores adsorbed on supported lipid bilayers provides visualization and additional quantification of the strength of dye-lipid interaction in the context of single-molecule measurements. By dissecting fluorophore-lipid interactions, our study provides new insights into setting up single-molecule fluorescence spectroscopy experiments with minimal interference from interactions between fluorescent labels and lipids in the environment.

Abstract B220: Artificially inducing protein-membrane anchorage: Introducing a new therapeutic modality

We aim to develop an innovative therapeutic modality of inhibiting aberrant protein function through suppression of activation and/or nuclear translocation. Nature has developed prenylation, a post-translational modification that covalently attaches a hydrophobic prenyl group to a protein to facilitate protein-membrane association to the plasma membrane. We hypothesize that mimicking Nature by artificially inducing protein-membrane anchorage through the use of a rationally designed protein-membrane anchor (PMA), we can simultaneously inhibit the activation and nuclear translocation of oncogenic proteins. Our aim is to explore the therapeutic potential of the protein-membrane anchor and potentially develop a novel drug modality that can be utilized by cancer patients.

Our proof-of-concept PMA was design to target signal transducer and activator of transcription 3 (Stat3) protein.

Constitutively-active Stat3 directly contributes to the progression of cancer and is present in numerous human cancers. A number of studies have shown that down-regulation of this oncogene via iRNA knockdown induces cellular apoptosis. Thus, Stat3 is an attractive target for the development of potent anti-cancer therapeutics for cancer.

Our proto-type PMA 1 was composed of two binding modules: a recognition motif to bind the protein and an anchor to sequester the protein complex to the membrane. The PMA was comprised of a potent Stat3 recognition sequence GpYLPQTV-NH2 covalently attached to a cholesterol membrane anchor.

We tested the ability of our PMA to anchor Stat3 to the cell membrane in MDA-MB-231 breast cancer cells which are known to have constitutively-active Stat3. We immunostained these cells with membrane stain FM-4–64 (red), anti-Stat3 antibody (green) and DAPI (nucleus, blue). In the absence of PMA 1, there was strong Stat3 nuclear presence. Most excitingly, in the presence of 25 μM concentration PMA 1, we observed complete sequestration of Stat3 to the cell membrane through PMA-Stat3 association. Currently, we are designing and synthesizing more drug-like, nonphosphorylated PMAs that are less prone to metabolic degradation. We will conduct further studies to determine the biochemical and biological utility of these PMAs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B220.

Coordination complex SH2 domain proteomimetics: an alternative approach to disrupting oncogenic protein-protein interactions

We report the first application of coordination complexes as functional proteomimetics of the Src homology 2 (SH2) phosphopeptide-binding domain. As a proof-of-concept, functionalized bis-dipicolylamine (BDPA) copper(ii) complexes are shown to disrupt oncogenic Stat3-Stat3 protein complexes and elicit promising anti-tumour activity.

Spectroscopic characterization of the spinach Lhcb4 protein (CP29), a minor light-harvesting complex of photosystem II

A spectroscopic characterization is presented of the minor photosystem II chlorophyll a/b-binding protein CP29 (or the Lhcb4 protein) from spinach, prepared by a modified form of a published protocol [Henrysson, T., Schroder, W. P., Spangfort, M. & Akerlund, H.-E. (1989) Biochim. Biophys. Acta 977, 301-308]. The isolation procedure represents a quicker, cheaper means of isolating this minor antenna protein to an equally high level of purity to that published previously. The pigment-binding protein shows similarities to other related light-harvesting complexes (LHCs), including the bulk complex LHCIIb but more particularly another minor antenna protein CP26 (Lhcb5). It is also, in the main, similar to other preparations of CP29, although some significant differences are discussed. In common with CP26, the protein binds about six chlorophyll a and two chlorophyll b molecules. Two chlorophyll b absorption bands are present at 638 and 650 nm and they are somewhat more pronounced than in a recent report [Giuffra, E., Zucchelli, G., Sandonà, D., Croce, R., Cugini, D., Garlaschi, F.M., Bassi, R. & Jennings, R.C. (1997) Biochem. 36, 12984-12993]. The bands give rise to positive and negative linear dichroism, respectively; both show negative CD bands (cf. bands with similar properties at 637 and 650 nm in CP26). Chlorophyll a absorption is dominated by a large contribution at 674 nm which also shows similarities to the major band in LHCIIb and CP26, while (as for CP26) a reduction in absorption around 670 nm is observed relative to the bulk complex. Principal differences from LHCIIb and CP26, and from other CP29 preparations, occur in the carotenoid region.