As a step toward studying membrane fusion with a simplified molecule, the ectodomain, residues 1-185, of the membrane-anchored subunit HA2 of the influenza virus haemagglutinin (HA) was solubilized by adding the very polar FLAG octapeptide (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) to the N-terminal HA2 fusion peptide. The resulting chimeric protein, F185, when expressed in bacteria, folded spontaneously into a soluble trimer, with a high alpha-helical content and a high melting temperature, structural characteristics of the low-pH-induced conformation of HA2. Removal of the FLAG octapeptide by proteolysis with enterokinase converted the soluble molecule to one that aggregated, bound nonionic detergent, and bound to lipid vesicles, properties of the low-pH-induced conformation of HA. Thermolysin treatment of the aggregated protein removed the nonpolar fusion peptide, regenerating soluble trimers of HA2 (residues 24-185), which is analogous to thermolysin treatment of HA in the low-pH-induced conformation. Thermolysin treatment also dissociates F185 from the detergent-protein complex by removing the fusion peptide. These results suggest that highly polar peptides can be fused to the membrane-binding regions of membrane proteins to increase their solubility. They also indicate that ectodomains of HA2 made in bacteria have membrane-binding properties similar to those of the same ectodomain generated by low-pH treatment of HA isolated from virus.