Scientific Report 2005

Molecular Biology

Lipid Chemistry for Studies of Integral Membrane Proteins

Q. Zhang, M.G. Finn,* X. Ma

* Department of Chemistry, Scripps Research

Integral membrane proteins float in the lipid bilayer with their hydrophobic domains threaded through the membrane and their hydrophilic domains extended into the aqueous solution. These proteins are extremely unstable outside the hydrophobic membrane bilayer, a situation that makes their in vitro biophysical and structural characterization difficult. An artificial environment is therefore needed to stabilize the proteins in their native state. We are attempting to synthesize new amphiphilic molecules that can extract integral membrane proteins from membranes and stabilize the proteins for structural characterization.

Relatively few investigators have actually addressed questions about the design of appropriate amphiphilic molecules despite the extensive use of such molecules in studies of membrane proteins. The criteria that we apply to generate such amphiphilic molecules are based on the physical properties of the molecules and on their interactions with membrane proteins. Detergents that self-assemble into micellar structures are universally used to dissolve integral membrane proteins as single particles to facilitate protein crystallization. We intend to incorporate more hydrophobicity in the interior of detergent micelles to improve the stability of the micelles and consequently their ability to stabilize integral membrane proteins. We accomplish this incorporation by appending branches along the alkyl chains of detergents and, most interestingly, by adding a short branch at the interface between the hydrophobic tail and the hydrophilic head. These branches may behave in 2 distinct ways like small amphiphile additives successfully used in crystallization of integral membrane proteins, thereby decreasing the micellar radius and extruding water from the hydrophobic core of the micelles.

The effect of these modifications on detergent micelle properties and on the stabilization and crystallization of integral membrane proteins is being investigated in collaboration with members of the Center for Innovative Membrane Protein Technologies of the Joint Center for Structural Genomics at Scripps Research. We are also interested in synthesizing additional novel amphiphilic molecules, including peptides, fluorinated lipids, and polymers that have special properties to facilitate the structural and functional study of integral membrane proteins.