Aqueous two-phase affinity partitioning of biotinylated liposomes using neutral avidin as affinity ligand

Purification of caveolae by affinity two-phase partitioning using biotinylated antibodies and NeutrAvidin–dextran

A new concept for affinity two-phase partitioning was tested. The partitioning was based on the interaction of target membranes with a primary antibody which, in turn, interacted with a biotinylated secondary antibody and NeutrAvidin-dextran in a poly(ethylene glycol)/dextran two-phase system. Caveolae selectively redistributed from the top phase to the NeutrAvidin-dextran-containing bottom phase by employing anti-caveolin as the primary antibody. This immunoaffinity approach was more selective than the established sucrose gradient centrifugation method and resulted in highly purified caveolae from Triton X-100-treated liver and lung plasma membranes. The same approach, employing other selective primary antibodies, should facilitate the purification also of other membrane fractions.

Affinity Two-Phase Partitioning in Acoustically Levitated Drops

Miniaturized (<1 microL) biospecific affinity two-phase partitioning in an acoustically levitated drop is described. Miniaturization commonly gives unfavorable surface/volume ratios, but in the levitation approach adsorption problems are minimized since the only surrounding wall is the liquid/air interface of the drop. Biotinylated liposomes were partitioned in aqueous poly(ethylene glycol)/dextran two-phase drops with NeutrAvidin-dextran as the affinity ligand. A two-phase drop was trapped and manipulated in a node of a standing ultrasonic wave. Alternatively, a two-phase system was formed by levitation and evaporation of a polymer one-phase drop. Phase mixing was achieved by adjusting the ultrasonic field and phase separation by readjusting the field. NeutrAvidin-dextran brought about the redistribution of biotinylated liposomes from the poly(ethylene glycol)-rich phase into the dextran-rich phase. Thus, an entire affinity two-phase separation procedure, including mixing of the phases and incubation to allow affinity interactions to develop under constant volume, followed by phase separation under controlled evaporation, can be performed in a single levitated drop. This miniaturized technique would allow the separation of biologically active membranes or organelles from individual cells for analysis.

Affinity partitioning for membrane purification exploiting the biotin-NeutrAvidin interaction. Model study of mixed liposomes and membranes

Biotinylated negatively charged liposomes as well as membranes were affinity partitioned in an aqueous poly(ethylene glycol)-dextran two-phase system using NeutrAvidin conjugated to dextran as affinity ligand. Both liposomes and membranes redistributed from top to bottom phase upon addition of NeutrAvidin-dextran. The presence of 35-60 mM Li2SO4 was necessary both to force the components into the top phase without ligand and for ligand-dependent redistribution into the bottom phase. Attaching biotin via a hexanamidohexanoyl spacer and an increased density of biotin or NeutrAvidin enhanced the affinity separation. The separation conditions in these model experiments provide a basis for affinity partitioning of membranes using other affinity ligands.

Affinity partitioning of biotinylated mixed liposomes: effect of charge on biotin--NeutrAvidin interaction

The partitioning behaviour of biotinylated mixed liposomes in aqueous poly(ethylene glycol)/dextran two-phase systems containing NeutrAvidin-dextran suggests that the biotin-NeutrAvidin affinity interaction is charge dependent. Biotinylated phosphatidylcholine liposomes with a low negative surface charge distributed in the NeutrAvidin-containing bottom phase at neutral pH, but the introduction of additional negative charges by including phosphatidylserine or the surfactant sodium dodecylsulfate in the liposomes caused them to distribute in the poly(ethylene glycol)-rich top phase instead. By gradually lowering the pH of the affinity two-phase system below the isoelectric point (6.3) of NeutrAvidin, negatively charged phosphatidylserine/phosphatidylcholine liposomes increasingly were attracted by NeutrAvidin to the bottom phase. It is suggested that acidic amino acids present at the rim of the biotin-binding pocket of NeutrAvidin may interact electrostatically with charged residues of the closely apposed liposome surface affecting the affinity interaction.