Conservation of surfactant protein A: evidence for a single origin for vertebrate pulmonary surfactant

Surface tension is reduced at the air-liquid interface in the lung by a mixture of lipids and proteins termed pulmonary surfactant. This study is the first to provide evidence for the presence of a surfactant-specific protein (Surfactant Protein A-SP-A) in the gas-holding structures of representatives of all the major vertebrate groups. Western blot analysis demonstrated cross-reactivity between an antihuman SP-A antibody and material lavaged from lungs or swimbladders of members from all vertebrate groups. Immunocytochemistry localized this SP-A-like protein to the air spaces of lungs from the actinopterygiian fish and lungfish. Northern blot analysis indicated that regions of the mouse SP-A cDNA sequence are complementary to lung mRNA from all species examined. The presence of an SP-A-like protein and SP-A mRNA in members of all the major vertebrate groups implies that the surfactant system had a single evolutionary origin in the vertebrates. Moreover, the evolution of the surfactant system must have been a prerequisite for the evolution of airbreathing. The presence of SP-A in the goldfish swimbladder demonstrates a role for the surfactant system in an organ that is no longer used for airbreathing.