Endocytosed SP-A and surfactant lipids are sorted to different organelles in rat type II pneumocytes

Intracellular transport of endocytosed surfactant protein A (SP-A) and lipi d was investigated in isolated rat type II cells. After internalization, SP -A and lipid are taken up via the coated-pit pathway and reside in a common compartment, positive for the early endosomal marker EEA1 but negative for the lamellar body marker 3C9. SP-A then recycles rapidly to the cell surfa ce via Rab4-associated recycling vesicles. Internalized lipid is transporte d toward a Rab7-, CD63-, 3C9-positive compartment, i.e., lamellar bodies. I nhibition of calmodulin led to inhibition of uptake and transport out of th e EEA1-positive endosome and thus of resecretion of both components. Inhibi tion of intravesicular acidification (bafilomycin A(1)) led to decreased up take of both surfactant components. It inhibited transport out of early end osomes for lipid only, not for SP-A. We conclude that in type II cells, end ocytosed SP-A and lipid are transported toward a common early endosomal com partment. Thereafter, both components dissociate. SP-A is rapidly recycled to the cell surface and does not enter classic lamellar bodies. Lipid is tr ansported toward lamellar bodies.