Peptides derived from apoptotic Bax and Bid reproduce the poration activity of the parent full-length proteins

¹ University of Valencia
² ITC-CNR Institute of Biophysics, Sezione di Trento
³ ITC & CNR-Istituto di Biofisica, Sezione di Trento

^* To whom correspondence should be addressed. E-mail: jesus.salgado{at}uv.es.

Submitted on December 30, 2004
Revised on January 30, 2005
Accepted on 4 March 2005

	Abstract

Bax and Bid are proapoptotic proteins of the Bcl-2 family that regulate the release of apoptogenic factors from mitochondria. Although they localize constitutively in the cytoplasm, their apoptotic function is exerted at the mitochondrial outer membrane, and is related to their ability to form transbilayer pores. Here we report the poration activity of fragments from these two proteins, containing the first -helix of a colicin like hydrophobic hairpin (-helix 5 of Bax and -helix 6 of Bid). Both peptides readily bind to synthetic lipid vesicles, where they adopt predominantly -helical structures and induce the release of entrapped calcein. In planar lipid membranes they form ion conducting channels, which in the case of the Bax-derived peptide are characterized by a two stage pattern, a large conductivity and lipid-charge dependent ionic selectivity. These features, together with the influence of intrinsic lipid curvature on the poration activity and the existence of two helical stretches of different orientations for the membrane bound peptide, suggest that it forms mixed lipidic/peptidic pores of toroidal structure. In contrast, the assayed Bid fragment shows a markedly different behavior, characterized by the formation of discrete, step-like channels in planar lipid bilayers, as expected for a peptidic pore lined by a bundle of helices.

Key Words: Bcl-2 family, apoptosis, ion channel, membrane protein, toroidal pore, transmembrane

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