Biophysical studies of membrane proteins/peptides. Interaction with ...

interact differentially, depending on the distribution of polar and hydrophobic residues
on the protein and on the size of the hydrophobic lipid domain.
These changes in selectivities of association can lead to changes in the lateral
distribution of the membrane components in a similar manner as observed for lipid
mixtures. In this way, a heterogeneous distribution of lipids can drive membrane protein
heterogeneity and vice-versa. In fact, it is known that proteins and lipids are often
distributed in a heterogeneous manner in the membrane, and this is essential in ensuring
the localized character of some events in cellular membranes.
Drugs are another type of agents which often display affinity for lipid membranes.
The targets of several drugs are membrane proteins, and their action is frequently
expected to be exerted in the membrane environment. Therefore, interaction of drugs
with biomembrane components is of great biological relevance. In addition, for drugs
targeting proteins inside the cell, partition to the plasma membrane is the first step to
cell entry, and water/lipid partition coefficients are in this way, relevant parameters in
drug research.
It is clear that interactions between biomembrane components are used by cells to
mediate and localize diverse functions in the cell membrane. A better understanding of
these mechanisms is the first step in rationalizing biomembranes and their functions.
However, cellular membranes are often too intricate for the properties of some
membrane component interactions to be resolved, and a valuable alternative is the use
of membrane model systems. Here, the majority of the studies were carried out in large
unilamellar vesicles, with a diameter around 100 nm. This model system granted us a
simplified basis for the study of the interactions between membrane components, at the
same time maintaining all the fundamental properties of the biomembrane.
Fluorescence techniques are ideal tools to study lateral distribution of membranes.
By choosing an intrinsic fluorescent amino acid (Trp or Tyr), or fluorescent labelling of
proteins and lipids, we can monitor fluorescence changes (intensity, lifetime,
anisotropy) upon interaction with other membrane components. Fluorescence quenching
studies can give information on the collision efficiency, and this can give us indications
on: i) the location of the fluorescent group; ii) on the concentration of quencher; and iii)
on the diffusion properties of both quencher and fluorescent molecule. One technique
that was particularly useful in evaluating lateral distributions in the membrane was
Förster resonance energy transfer (FRET). FRET efficiency for a single donor-acceptor
pair is strongly dependent on the distance between the two. In the case of lipid