Conformational Modulation in the Protein L-BABP by Anionic Lipid Membranes: A Tryptophan Fluorescence Study

VANESA GALASSI, GUILLERMO MONTICH

Buzios, Brasil

Congreso; VII Iberoamerican Congress of Biophysics; 2009

Sociedade Brasileira de Biofísica, Sociedad de Biofísicos Latinoamericanos, Sociedad Argentina de Biofísica, Federación Latinoamerica de Sociedades de Biofísica, Sociedade Portuguesa de Biofísica y la Sociedad Española de Biofísica

The regulation of protein conformation by lipid membrane binding is fundamental in many cellular functions. In many pathologies related to protein misfolding, such as prion or amiloid-like, the conformational transition is triggered by the association to the lipid interface. We used L-BABP as a model protein to study this conformational regulation. We performed fluorescence spectroscopy experiments to characterize this effect. In agreement to infrared experiments, we observed that when this protein binds to anionic membranes, it suffers a partial unfolding that affects its thermal stability. In solution, it unfolds and aggregates at 70ºC, while when associated to a lipid membrane, the denaturation is shifted to lower temperatures, up to 40ºC , as was revealed by Trp6 fluorescence anisotropy and differential scanning calorimetry. According to Molecular Dynamic simulations, partial unfolding upon membrane binding involves the upper part of the beta-barrel increasing the water accessibility to the inside of the cavity where Trp6 is located. Accordingly, we observed a batochromic shift up to 10 nm, sensitive to the degree of unfolding. Our results suggests that the partly-unfolded protein actually consists of an ensemble of states with different degrees of folding and the ionic strength affects both the membrane binding and the distribution of these states. This distribution also depends on membrane properties: lipid phase and packing degree. We also conclude that the conformational modulation is primarily due to the surface electrostatic potential.