Blood Journal
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Roles of RabGEF1/Rabex-5 domains in regulating FcϵRI surface expression and FcϵRI-dependent responses in mast cells

  1. Janet Kalesnikoff1,
  2. Eon J. Rios1,
  3. Ching-Cheng Chen1,
  4. M. Alejandro Barbieri2,
  5. Mindy Tsai1,
  6. See-Ying Tam1, and
  7. Stephen J. Galli1
  1. 1Department of Pathology, Stanford University School of Medicine, Stanford, CA;
  2. 2Department of Biological Sciences, Florida International University, College of Arts and Sciences, Miami, FL

Abstract

RabGEF1/Rabex-5, a guanine nucleotide exchange factor (GEF) for the endocytic pathway regulator, Rab5, contains a Vps9 domain, an A20-like zinc finger (ZnF) domain, and a coiled coil domain. To investigate the importance of these domains in regulating receptor internalization and cell activation, we lentivirally delivered RabGEF1 mutants into RabGEF1-deficient (−/−) mast cells and examined FcϵRI-dependent responses. Wild-type RabGEF1 expression corrected phenotypic abnormalities in −/− mast cells, including decreased basal FcϵRI expression, slowed FcϵRI internalization, elevated IgE + Ag–induced degranulation and IL-6 production, and the decreased ability of −/− cytosol to support endosome fusion. We showed that RabGEF1's ZnF domain has ubiquitin ligase activity. Moreover, the coiled coil domain of RabGEF1 is required for Rabaptin-5 binding and for maintaining basal levels of Rabaptin-5 and surface FcϵRI. However, mutants lacking either of these domains normalized phenotypic abnormalities in IgE + antigen–activated −/− mast cells. By contrast, correction of these −/− phenotypes required a functional Vps9 domain. Thus, FcϵRI-mediated mast cell functional activation is dependent on RabGEF1's GEF activity.

  • Submitted January 10, 2007.
  • Accepted February 26, 2007.
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