Munc13-4 is an effector of rab27a and controls secretion of lysosomes in hematopoietic cells

Mol Biol Cell. 2005 Feb;16(2):731-41. doi: 10.1091/mbc.e04-10-0923. Epub 2004 Nov 17.


Griscelli syndrome type 2 (GS2) is a genetic disorder in which patients exhibit life-threatening defects of cytotoxic T lymphocytes (CTLs) whose lytic granules fail to dock on the plasma membrane and therefore do not release their contents. The disease is caused by the absence of functional rab27a, but how rab27a controls secretion of lytic granule contents remains elusive. Mutations in Munc13-4 cause familial hemophagocytic lymphohistiocytosis subtype 3 (FHL3), a disease phenotypically related to GS2. We show that Munc13-4 is a direct partner of rab27a. The two proteins are highly expressed in CTLs and mast cells where they colocalize on secretory lysosomes. The region comprising the Munc13 homology domains is essential for the localization of Munc13-4 to secretory lysosomes. The GS2 mutant rab27aW73G strongly reduced binding to Munc13-4, whereas the FHL3 mutant Munc13-4Delta608-611 failed to bind rab27a. Overexpression of Munc13-4 enhanced degranulation of secretory lysosomes in mast cells, showing that it has a positive regulatory role in secretory lysosome fusion. We suggest that the secretion defects seen in GS2 and FHL3 have a common origin, and we propose that the rab27a/Munc13-4 complex is an essential regulator of secretory granule fusion with the plasma membrane in hematopoietic cells. Mutations in either of the two genes prevent formation of this complex and abolish secretion.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Blotting, Western
  • Cell Line
  • Glutathione Transferase / metabolism
  • Green Fluorescent Proteins / metabolism
  • HeLa Cells
  • Humans
  • Immunohistochemistry
  • Jurkat Cells
  • K562 Cells
  • Lysosomes / metabolism*
  • Mast Cells / cytology*
  • Mast Cells / metabolism*
  • Mast Cells / ultrastructure
  • Microscopy, Immunoelectron
  • Mutation
  • Protein Binding
  • Protein Structure, Tertiary
  • Proteins / chemistry
  • Proteins / genetics
  • Proteins / metabolism*
  • Proteins / ultrastructure
  • Rats
  • Recombinant Proteins / metabolism
  • Sulfur Radioisotopes / metabolism
  • T-Lymphocytes, Cytotoxic / metabolism
  • Transfection
  • U937 Cells
  • rab GTP-Binding Proteins / chemistry
  • rab GTP-Binding Proteins / genetics
  • rab GTP-Binding Proteins / metabolism*
  • rab27 GTP-Binding Proteins


  • Proteins
  • Recombinant Proteins
  • Sulfur Radioisotopes
  • Unc13d protein, rat
  • rab27 GTP-Binding Proteins
  • Green Fluorescent Proteins
  • Glutathione Transferase
  • RAB27A protein, human
  • Rab27a protein, rat
  • rab GTP-Binding Proteins