Submitted April 22, 2008
Accepted July 21, 2008
Rabaptin-5 regulates receptor expression and functional activation in mast cells
Eon J Rios, Adrian M Piliponsky, Chisei Ra, Janet Kalesnikoff, and Stephen J. Galli*
Department of Pathology, Stanford University School of Medicine, Stanford, CA, United States
Department of Molecular Cell Immunology and Allergology, Nihon University Graduate School of Medical Science, Tokyo, Japan
* Corresponding author; email: sgalli{at}stanford.edu.
Rab5 is a small GTPase that regulates early endocytic events and is activated by RabGEF1/Rabex-5. Rabaptin-5, a Rab5 interacting protein, was identified as a protein critical for potentiating RabGEF1/Rabex-5's activation of Rab5. Using Rabaptin-5 shRNA knockdown, we show that Rabaptin-5 is dispensable for Rab5-dependent processes in intact mast cells, including high affinity IgE receptor (Fc
RI) internalization and endosome fusion. However, Rabaptin-5 deficiency markedly diminished expression of FcRI and 
1 integrin on the mast cell surface by diminishing receptor surface stability. This in turn reduced the ability of mast cells to bind IgE and significantly diminished both mast cell sensitivity to antigen (Ag)-induced mediator release and Ag-induced mast cell adhesion and migration. These findings show that although dispensable for canonical Rab5 processes in mast cells, Rabaptin-5 importantly contributes to mast cell IgE-dependent immunological function by enhancing mast cell receptor surface stability.
