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J. Biol. Chem., Vol. 278, Issue 6, 3929-3936, February 7, 2003

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Reconstitution of Holin Activity with a Synthetic Peptide Containing the 1-32 Sequence Region of EJh, the EJ-1 Phage Holin^*

From the ^a Insto de Química-Física Rocasolano and ^g Centro de Biología Molecular Severo Ochoa, Consejo Superior de Investigaciones Científicas, Madrid 28006, Spain, the ^c Facultad de Ciencias C-XVI, University Autónoma, Madrid 28049, Spain, ^d Structure and Function of Biological Membranes, University Libre Bruxelles, Bruxelles B1050, Belgium, the ^f Facultad Ciencias Experimentales, University Pablo de Olavide, Sevilla 41013, Spain, and the ^h Departamento Ciencias Experimentales y de la Salud, University Pompeu Fabra, Barcelona 08003, Spain

Pneumococcal EJ-1 phage holin (EJh) is a hydrophobic polypeptide of 85 amino acid residues displaying lethal inner membrane disruption activity. To get an insight into holin structure and function, several peptides representing the different topological regions predicted by sequence analysis have been synthesized. Peptides were structurally characterized in both aqueous buffer and membrane environments, and their potential to induce membrane perturbation was determined. Among them, only the N-terminal predicted transmembrane helix increased the membrane permeability. This segment, only when flanked by the positive charged residues on its N-terminal side, which are present in the sequence of the full-length protein, folds into a major -helix structure with a transmembrane preferential orientation. Fluorescein quenching experiments of N-terminal-labeled peptide evidenced the formation of oligomers of variable size depending on the peptideto-lipid molar ratio. The self-assembling tendency correlated with the formation of transmembrane pores that permit the release of encapsulated dextrans of various sizes. When analyzed by atomic force microscopy, peptide-induced membrane lesions are visualized as transbilayer holes. These findings are the first evidence for a lytic domain in holins and for the nature of membrane lesions caused by them.

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