Background & aims: Glycolipid "rafts" have been shown to play a role in apical membrane trafficking in the enterocyte. The present study characterized the membrane compartments of the enterocyte involved in transepithelial transport of small intestinal immunoglobulin A (IgA).
Methods: Immunogold electron microscopy and radioactive labeling of mouse small intestinal explants were performed.
Results: IgA and the polymeric immunoglobulin receptor/secretory component were present in a raft compartment. Raft association occurred posttranslationally within 30 minutes, preceding secretion into the culture medium. IgA labeling was seen primarily in enterocytes along the basolateral plasma membrane and over endosomes and small vesicles in the basolateral and apical regions of the cytoplasm. IgA and a brush border enzyme, aminopeptidase N, were colocalized in apical endosomes and small vesicles and were also frequently seen associated with the same vesicular profiles of glycolipid rafts. Colocalization of IgA and rab17, a small guanosine triphosphatase involved in transcytosis, was seen mainly along the basolateral plasma membrane and over basolateral endosomes and vesicles, but also in the apical region of the cytoplasm.
Conclusions: IgA is transcytosed through a raft-containing compartment, most likely the apical endosomes. Our data also support the notion that rab17 is involved in transcytotic membrane traffic.