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Fig. 2. Rab27a regulates the localization and exocytosis of lysosome-related organelles in different cell types through distinct effector proteins. Mutations in Rab27a and its interaction partners cause several diseases characterized by defects in the subcellular distribution or exocytosis of lysosome-related organelles (A) Pigmentation defects involve mutations in Rab27a, myosin 5a or melanophilin, which bridges the former two proteins. This leads to disturbance of tethering and local movement of melanosomes at the distal actin-rich regions of the melanocyte. (B) Immunological deficits involve defects in Rab27a or its effector Munc13-4. The Rab27a defects disturb the targeting of secretory lysosomes of cytotoxic T-cells to the immunological synapse, whereas Munc13-4 defects apparently disturb the priming of lysosomes for fusion with the plasma membrane. The function of Munc13-4 is thought to be connected with that of the SNARE-based fusion machinery. Components with identified disease mutations are indicated by *.
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