Syntaxins 13 and 7 function at distinct steps during phagocytosis

The phagosome is a dynamic organelle that undergoes progressive changes to acquire the machinery required to kill and degrade internalized foreign particles. This maturation process involves sequential interaction of newly formed phagosomes with several components of the endocytic pathway. The proteins that mediate the ordered fusion of endosomes and lysosomes with the phagosome are not known. In this study, we investigated the possible role of syntaxins present in the endo/lysosomal pathway in directing phagosomal maturation. We show that in phagocytic cells syntaxin 13 is localized to the recycling endosome compartment, while syntaxin 7 is found in late endosomes/lysosomes. Both proteins are recruited to the phagosome, but syntaxin 13 is acquired earlier and rapidly recycles off the phagosome, while syntaxin 7 is recruited later and continues to accumulate throughout the maturation process. Overexpression of truncated (cytosolic) forms of syntaxin 13 or 7 had no effect on phagocytosis, but exerted an inhibitory effect on phagosomal maturation. These results indicate that syntaxins 13 and 7 are both required for interaction of endosomes and/or lysosomes with the phagosome, but play distinct roles in the maturation process.