GALA is a 30 amino acid synthetic peptide with a glutamic acid-alanine-leucine-alanine (EALA) repeat that also contains a histidine and tryptophan residue as spectroscopic probes. It was designed to explore how viral fusion protein sequences interact with membranes. The sequence selected was long enough to span a bilayer in the alpha-helix, the glutamic acids (Glu) were selected to provide a pH-dependent negatively charged side-chain and the EALA repeat was adjusted so that the peptide would have a hydrophobic face of sufficient hydrophobicity to interact with the bilayer when the peptide was in an alpha-helix. GALA converts from a random coil to an amphipathic alpha-helix when the pH is reduced from 7.0 to 5.0. At neutral pH, GALA is water soluble while at acid pH, GALA binds to bilayer membranes. The nature of the association and the type of peptide-peptide interactions in the membrane depend upon the physico-chemical properties of the bilayer such as the acyl chain composition of the phospholipids and the presence of cholesterol. Neutral and negatively charged bilayers composed of saturated phospholipids of 14-16 acyl chain length are solubilized into peptide-lipid discs by GALA. GALA can induce fusion between small unilamellar vesicles (SUV) composed of unsaturated phospholipids. Most importantly GALA forms a transmembrane peptide pore comprised of approximately 10 GALA alpha-helical monomers that are arrayed in an alpha-helix perpendicular to the plane of the membrane. Membrane leakage from neutral or negatively charged vesicles at pH 5.0 can be adequately explained by a mathematical model assuming that GALA becomes incorporated into the vesicle bilayer and aggregates to form a transbilayer pore consisting of 10 (+/-2) peptides. The lipid compositions of model bilayer have important effects on the GALA transbilayer insertion mechanism and peptide orientation. Insertion of the pore into the membrane dramatically accelerates transmembrane phospholipid flip-flop. Cationic peptides designed based upon GALA but containing a lysine-alanine-leucine-alanine (KALA) motif can interact with nucleic acids and perturb biomembranes. The pH-controlled membrane permealization induced by GALA and related peptides serve as a paradigm for the design of environmentally responsive peptidic delivery vehicles for drugs and genes.