Flagellar membrane localization via association with lipid rafts.

The eukaryotic flagellar membrane has a distinct composition from other domains of the plasmalemma. Our work shows that the specialized composition of the trypanosome flagellar membrane reflects increased concentrations of sterols and saturated fatty acids, correlating with direct observation of high liquid order by laurdan fluorescence microscopy. These findings indicate that the trypanosome flagellar membrane possesses high concentrations of lipid rafts: discrete regions of lateral heterogeneity in plasma membranes that serve to sequester and organize specialized protein complexes. Consistent with this, a dually acylated Ca(2+) sensor that is concentrated in the flagellum is found in detergent-resistant membranes and mislocalizes if the lipid rafts are disrupted. Detergent-extracted cells have discrete membrane patches localized on the surface of the flagellar axoneme, suggestive of intraflagellar transport particles. Together, these results provide biophysical and biochemical evidence to indicate that lipid rafts are enriched in the trypanosome flagellar membrane, providing a unique mechanism for flagellar protein localization and illustrating a novel means by which specialized cellular functions may be partitioned to discrete membrane domains.

Genetically encoded probe for fluorescence lifetime imaging of CaMKII activity.

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is highly enriched in excitatory synapses in the central nervous system and is critically involved in synaptic plasticity, learning, and memory. However, the precise temporal and spatial regulation of CaMKII activity in living cells has not been well described, due to lack of a specific method. Here, based on our previous work, we attempted to generate an optical probe for fluorescence lifetime imaging (FLIM) of CaMKII activity by fusing the protein with donor and acceptor fluorescent proteins at its amino- and carboxyl-termini. We first optimized the combinations of fluorescent proteins by taking advantage of expansion of fluorescent proteins towards longer wavelength in fluorospectrometric assay. Then using digital frequency domain FLIM (DFD-FLIM), we demonstrated that the resultant protein can indeed detect CaMKII activation in living cells. These FLIM versions of Camui could be useful for elucidating the function of CaMKII both in vitro and in vivo.

Structure and dynamics of the epidermal growth factor receptor C-terminal phosphorylation domain.

The C-terminal phosphorylation domain of the epidermal growth factor receptor is believed to regulate protein kinase activity as well as mediate the assembly of signal transduction complexes. The structure and dynamics of this proposed autoregulatory domain were examined by labeling the extreme C terminus of the EGFR intracellular domain (ICD) with an extrinsic fluorophore. Fluorescence anisotropy decay analysis of the nonphosphorylated EGFR-ICD yielded two rotational correlation times: a longer time, consistent with the global rotational motion of a 60- to 70-kDa protein with an elongated globular conformation, and a shorter time, presumably contributed by segmental motion near the fluorophore. A C-terminally truncated form of EGFR-ICD yielded a slow component consistent with the rotational motion of the 38-kDa kinase core. These findings suggested a structural arrangement of the EGFR-ICD in which the C-terminal phosphorylation domain interacts with the kinase core to move as an extended structure. A marked reduction in the larger correlation time of EGFR-ICD was observed upon its autophosphorylation. This dynamic component was faster than predicted for the globular motion of the 62-kDa EGFR-ICD, suggesting an increase in the mobility of the C-terminal domain and a likely displacement of this domain from the kinase core. The interaction between the SH2 domain of c-Src and the phosphorylated EGFR C-terminal domain was shown to impede its mobility. Circular dichroism spectroscopy indicated that the EGFR C-terminal domain possessed a significant level of secondary structure in the form of alpha-helices and beta-sheets, with a marginal change in beta-sheet content occurring upon phosphorylation.

Application of fluorescence correlation spectroscopy to hapten-antibody binding.

Two-photon fluorescence correlation spectroscopy 2P-FCS has received a large amount of attention over the past ten years as a technique that can monitor the concentration, the dynamics, and the interactions of molecules with single molecule sensitivity. In this chapter, we explain how 2P-FCS is carried out for a specific ligand-binding problem. We briefly outline considerations for proper instrument design and instrument calibration. General theory of autocorrelation analysis is explained and straightforward equations are given to analyze simple binding data. Specific concerns in the analytical methods related to IgG, such as the presence of two equivalent sites and fractional quenching of the bound hapten-fluorophore conjugate, are explored and equations are described to account for these issues. We apply these equations to data on two antibody-hapten pairs: antidigoxin IgG with fluorescein-digoxin and antidigitoxin IgG with Alexa488-digitoxin. Digoxin and digitoxin are important cardio glycoside drugs, toxic at higher levels, and their blood concentrations must be monitored carefully. Clearly, concentration assays based on IgG rely on accurate knowledge of the hapten-IgG binding strengths. The protocols for measuring and determining the dissociation constants for both IgG-hapten pairs are outlined and discussed.

Visualization and analysis of apolipoprotein A-I interaction with binary phospholipid bilayers.

Apolipoprotein A-I (apoA-I) interaction with specific cell lipid domains was suggested to trigger cholesterol and phospholipid efflux. We analyzed here apoA-I interaction with dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) bilayers at a temperature showing phase coexistence. Solid and liquid-crystalline domains were visualized by two-photon fluorescence microscopy on giant unilamellar vesicles (GUVs) labeled with 6-dodecanoyl-2-dimethyl-amino-naphthalene (Laurdan). A decrease of vesicle size was detected as long as they were incubated with lipid-free apoA-I, together with a shape deformation and a relative enrichment in DSPC. Selective lipid removal mediated by apoA-I from different domains was followed in real time by changes in the Laurdan generalized polarization. The data show a selective interaction of apoA-I with liquid-crystalline domains, from which it removes lipids, at a molar ratio similar to the domain compositions. Next, apoA-I was incubated with DMPC/DSPC small unilamellar vesicles, and products were isolated and quantified. Protein solubilized both lipids but formed complexes relatively enriched in the liquid component. We also show changes in the GUV morphology when cooling down. Our results suggest that the most efficient reaction between apoA-I and DMPC/DSPC occurs in particular bilayer conditions, probably when small fluid domains are nucleated within a continuous gel phase and interfacial packing defects are maximal.

Surface-exposed tryptophan residues are essential for O-acetylserine sulfhydrylase structure, function, and stability.

O-Acetylserine sulfhydrylase is a homodimeric enzyme catalyzing the last step of cysteine biosynthesis via a Bi Bi ping-pong mechanism. The subunit is composed of two domains, each containing one tryptophan residue, Trp50 in the N-terminal domain and Trp161 in the C-terminal domain. Only Trp161 is highly conserved in eucaryotes and bacteria. The coenzyme pyridoxal 5'-phosphate is bound in a cleft between the two domains. The enzyme undergoes an open to closed conformational transition upon substrate binding. The effect of single Trp to Tyr mutations on O-acetylserine sulfhydrylase structure, function, and stability was investigated with a variety of spectroscopic techniques. The mutations do not significantly alter the enzyme secondary structure but affect the catalysis, with a predominant influence on the second half reaction. The W50Y mutation strongly affects the unfolding pathway due to the destabilization of the intersubunit interface. The W161Y mutation, occurring in the C-terminal domain, produces a reduction of the accessibility of the active site to acrylamide and stabilizes thermodynamically the N-terminal domain, a result consistent with stronger interdomain interactions.

Relationship between erythrocyte membrane phase properties and susceptibility to secretory phospholipase A2.

Normally, cell membranes resist hydrolysis by secretory phospholipase A(2). However, upon elevation of intracellular calcium, the cells become susceptible. Previous investigations demonstrated a possible relationship between changes in lipid order caused by increased calcium and susceptibility to phospholipase A(2). To further explore this relationship, we used temperature as an experimental means of manipulating membrane physical properties. We then compared the response of human erythrocytes to calcium ionophore at various temperatures in the range of 20-50 degrees C using fluorescence spectroscopy and two-photon fluorescence microscopy. The steady state fluorescence emission of the environment-sensitive probe, laurdan, revealed that erythrocyte membrane order decreases systematically with temperature throughout this range, especially between 28 and 45 degrees C. Furthermore, the ability of calcium ionophore to induce increased membrane order and susceptibility to phospholipase A(2) depended similarly on temperature. Both responses to calcium influx were enhanced as membrane fluidity increased. Analysis of the spatial distribution of laurdan fluorescence at several temperatures indicated that the ordering effect of intracellular calcium on fluid membranes generates an increase in the number of fluid-solid boundaries. Hydrolysis of the membrane appeared to progress outward from these boundaries. We conclude that phospholipase A(2) prefers to hydrolyze lipids in fluid regions of human erythrocyte membranes, but primarily when those regions coexist with domains of ordered lipids.

Unfolding of pyridoxal 5'-phosphate-dependent O-acetylserine sulfhydrylase probed by time-resolved tryptophan fluorescence.

Proteins utilizing pyridoxal 5'-phosphate as a coenzyme constitute a large superfamily and are currently classified into three functional groups and five structural fold types. Despite the variability of sequences and catalyzed reactions, they share relevant structural, dynamic and functional properties. Therefore, they constitute an optimal system to investigate the relative influence of primary sequence and coenzyme interactions on folding pathways, structural stability and enzymatic function. O-Acetylserine sulfhydrylase is a dimeric pyridoxal 5'-phosphate dependent enzyme that catalyzes the synthesis of L-cysteine from O-acetylserine and sulfide. The time-resolved fluorescence study of O-acetylserine sulfhydrylase unfolding, here reported, indicates that the coenzyme stabilizes the protein structure. The dependence on denaturant concentration of tryptophan lifetimes in the holo- and apo-enzyme demonstrates that the interactions with the coenzyme stabilize the C-terminal domain to a higher extent with respect to the N-terminal domain. This result is discussed in terms of a linkage between the differential stabilization brought about by the coenzyme and the different degrees of conformational flexibility required by the specialized functional role of distinct protein regions.

A two-photon view of an enzyme at work: Crotalus atrox venom PLA2 interaction with single-lipid and mixed-lipid giant unilamellar vesicles.

We describe the interaction of Crotalus atrox-secreted phospholipase A2 (sPLA2) with giant unilamellar vesicles (GUVs) composed of single and binary phospholipid mixtures visualized through two-photon excitation fluorescent microscopy. The GUV lipid compositions that we examined included 1-palmitoyl-2-oleoyl-phosphatidylcholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) (above their gel-liquid crystal transition temperatures) and two well characterized lipid mixtures, 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE):DMPC (7:3) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC) (1:1) equilibrated at their phase-coexistence temperature regime. The membrane fluorescence probes, 6-lauroyl-2-(dimethylamino) napthalene, 6-propionyl-2-(dimethylamino) naphthalene, and rhodamine-phosphatidylethanolamine, were used to assess the state of the membrane and specifically mark the phospholipid domains. Independent of their lipid composition, all GUVs were reduced in size as sPLA2-dependent lipid hydrolysis proceeded. The binding of sPLA2 was monitored using a fluorescein-sPLA2 conjugate. The sPLA2 was observed to associate with the entire surface of the liquid phase in the single phospholipid GUVs. In the mixed-lipid GUV's, at temperatures promoting domain coexistence, a preferential binding of the enzyme to the liquid regions was also found. The lipid phase of the GUV protein binding region was verified by the introduction of 6-propionyl-2-(dimethylamino) naphthalene, which partitions quickly into the lipid fluid phase. Preferential hydrolysis of the liquid domains supported the conclusions based on the binding studies. sPLA2 hydrolyzes the liquid domains in the binary lipid mixtures DLPC:DAPC and DMPC:DMPE, indicating that the solid-phase packing of DAPC and DMPE interferes with sPLA2 binding, irrespective of the phospholipid headgroup. These studies emphasize the importance of lateral packing of the lipids in C. atrox sPLA2 enzymatic hydrolysis of a membrane surface.