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 stu dy is the first to provide evidence for the presence of a surfactant-s pecific protein (Surfactant Protein A-SP-A) in the gas-holding structu res of representatives of all the major vertebrate groups. Western blo t analysis demonstrated crossreactivity between an antihuman SP-A anti body and material lavaged from lungs or swimbladders of members from a ll vertebrate groups. Immunocytochemistry localized this SP-A-like pro tein to the air spaces of lungs from the actinopterygiian fish and lun gfish. Northern blot analysis indicated that regions of the mouse SP-A cDNA sequence are complementary to lung mRNA from all species examine d. The presence of an SP-A-like protein and SP-A mRNA in members of al l the major vertebrate groups implies that the surfactant system had a single evolutionary origin in the vertebrates. Moreover, the evolutio n of the surfactant system must have been a prerequisite for the evolu tion 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.