Interaction Between a Peptide Corresponding to Sticholysin II (StII) N-Terminus and Model Membranes. Implications for the Mechanism of Pore Formation
Joana Paulino1;Eduardo M. Cilli2;M. Eliana Lanio3; Carlos M. Alvarez1,3; and Shirley Schreier1
1Institute of Chemistry, USP, São Paulo; 2Institute of Chemistry, UNESP, Araraquara, Brazil; 3Center for Protein Studies, University of Havana, Havana, Cuba.
StII is a water soluble protein produced by the Caribbean sea anemone Stichodactyla heliantus that binds to membranes, causing lysis. Due to StII affinity for sphingomyelin (SM), this lipid has been postulated as its membrane receptor. However, the toxin mechanism of action is not fully understood at the molecular level. StII´s N-terminal region has been proposed to have an important role in pore formation. To examine this hypothesis, we studied the interaction between a peptide containing residues 1-30 (P1-30) of StII and small unilamelar vesicles (SUV), containing variable quantities of egg phosphatidylcholine (ePC), SM, and negatively charged phosphatidic acid (PA) by means of circular dichroism (CD). P1-30 aggregates in aqueous solution (pH 7) probably due to the highly hydrophobic segment in residues 1-10. Upon binding to membranes, disaggregation and increase of secondary structure content occurred. The peptide acquired a-helical conformation that increased with the increase of the SUV/peptide ratio. Addition of PA to ePC bilayers enhanced binding and caused an increase of a-helical conformation. This is due to electrostatic interactions between the positively charged peptide and the negatively charged membrane. Moreover, it is also possible that peptide binding is favored by PA-promoted negative curvature. When SM was added as the third component the binding of P1-30 increased. As a result, the peptide a-helical content also increased. The SM-induced enhanced binding could be due to phase separation in the SM-ePC-PA system, since it is known that SM tends to form a more organized phase than ePC and PA. This phenomenon could cause membrane defects and, thereby, facilitate insertion of the peptide hydrophobic region in the bilayer. The results clearly point to the importance of lipid composition, in particular, the presence of SM, in the interaction between StII N-terminus and model membranes. Acquisition of helical structure and peptide insertion in the membrane could represent the initial steps of pore formation.
Supported by FAPESP, CNPq.
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