Specific interactions between the human serotonin transporter and serotonin analogs at the solution/air interface

Surface Pressure Analysis of Poly(ethylene oxide)-Modified Fusogenic Liposomes Incorporated into a Phospholipid Monolayer

Fusogenic liposomes have a wide-range of applications as DDS and gene/protein delivery into living cells. A variety of surface modifications of drug carriers, to enable fusion with cells, have been proposed, however, the mechanism of fusion has still not been determined. To further improve the efficiency of drug carriers, a simple and easily examinable model of a living cell surface is needed. In this study, the time-course of a fusion phenomena was made by measuring the surface pressure increase of a phospholipid monolayer spread at the air/water interface due to the fusion of liposomes carrying PEO-lipid (dialkyl-terminated polyethylene oxide) reconstituted on their outer surface. The kinetics of the surface pressure change appeared to be bimodal, indicating the coexistence of different fusion pathways. It was found that the presence of the PEO-lipid on the liposome surface led to a faster lipid transfer compared to non-modified DMPC liposomes. This indicated that the reconstitution of PEO-lipid provided an alternative transfer pathway to that for non-fusogenic liposomes that show only a slow lipid transfer to phospholipid monolayers. The relation between the rate of fusion and the surface pressure of the host membrane is discussed.

Molecular recognition on the supported and on the air/water interface-spread protein monolayers

Targeting of proteins at interfaces via affinity ligands or specific antibodies is important for the understanding of protein functioning in biological membranes. This review brings together a great number of research works accomplished in this field in the past decade by a variety of analytical methods. It highlights two simple in situ techniques of monitoring molecular recognition processes at interfaces recently developed in the author's laboratory. The first of these techniques is based on the measurements of surface pressure increments of a protein monolayer spread at the air/water interface at a constant area resulting from the interaction with its specific ligands injected into the aqueous subphase beneath the preformed protein monolayer. The second technique takes advantage of the feature of [(14)C]-labeled proteins that enable in situ measurements of surface density changes of adsorbed protein molecules on a solid support resulting from the interaction with its specific antibody.

Molecular organization of the human serotonin transporter at the air/water interface

The serotonin transporter (SERT) is the target of several important antidepressant and psychostimulant drugs. It has been shown that under defined conditions, the transporter spread at the air/water interface was able to bind its specific ligands. In this paper, the interfacial organization of the protein has been assessed from dynamic surface pressure and ellipsometric measurements. For areas comprising between 10,400 and 7,100 A(2)/molecule, ellipsometric measurements reveal an important change in the thickness of the SERT film. This change was attributed to the reorientation of the transporter molecules from a horizontal to their natural predictive transmembrane orientation. The thickness of the SERT film at 7,100 A(2)/molecule was found to be approximately equal to 84 A and coincided well with the theoretical value estimated from the calculations based on the dimensions of alpha-helices containing membrane proteins. These data suggest that the three-dimensional arrangement of the SERT may be represented as a box with lengths d(z)=83--85 A and d(y) or d(x)=41--47 A.

Ligand interaction with the purified serotonin transporter in solution and at the air/water interface

The purified serotonin transporter (SERT) was spread at the air/water interface and the effects both of its surface density and of the temperature on its interfacial behavior were studied. The recorded isotherms evidenced the existence of a stable monolayer undergoing a lengthy rearrangement. SERT/ligand interactions appeared to be dependent on the nature of the studied molecules. Whereas an unrelated drug (chlorcyclizine) did not bind to the spread SERT, it interacted with its specific ligands. Compared to heterocyclic drugs, for which binding appeared to be concentration-dependent, a 'two-site' mechanism was evidenced for pinoline and imipramine.