The Monomeric Guanosine Triphosphatase rab4 Controls an Essential Step on the Pathway of Receptor-mediated Antigen Processing in B Cells

Each member of the rab guanosine triphosphatase protein family assists in the regulation of a specific step within the biosynthetic or endocytic pathways. We have found that the early endosome-associated rab4 protein controls a step critical for receptor-mediated antigen processing in a murine A20 B cell line. Expression of the dominant negative rab4N(121)I mutant dramatically inhibited the processing and presentation of ovalbumin, λ cI repressor, or rabbit immunoglobulin G internalized as antigens by B cell antigen receptors or transfected Fc receptors. This defect did not reflect a block in antigen endocytosis or degradation, and transfected cells remained completely capable of presenting exogenously added ovalbumin and λ repressor peptides. Most remarkably, rab4N(121)I-expressing cells were undiminished in their ability to present each of these antigens when whole proteins were internalized at high concentration by fluid-phase endocytosis. Thus, expression of the rab4N(121)I selectively inactivated a portion of the endocytic pathway required for the processing of receptor-bound, but not nonspecifically internalized, antigens. These results suggest that elements of the early endosome-recycling pathway play an important and selective role in physiologically relevant forms of antigen processing in B cells.