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Vol. 17, Issue 9, 3964-3977, September 2006

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Involvement of Syntaxin 18, an Endoplasmic Reticulum (ER)-localized SNARE Protein, in ER-mediated Phagocytosis

Kiyotaka Hatsuzawa^*,, Taku Tamura^*,, Hitoshi Hashimoto^*,, Hiromi Hashimoto^*,, Sachihiko Yokoya^*, Megumi Miura^*,, Hisao Nagaya^*,, and Ikuo Wada^*,

*Department of Cell Science, Institute of Biomedical Sciences, Fukushima Medical University School of Medicine, Fukushima 960-1295, Japan; and CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

Submitted December 30, 2005; Revised June 8, 2006; Accepted June 12, 2006
Monitoring Editor: Benjamin Glick

The endoplasmic reticulum (ER) is thought to play an important structural and functional role in phagocytosis. According to this model, direct membrane fusion between the ER and the plasma or phagosomal membrane must precede further invagination, but the exact mechanisms remain elusive. Here, we investigated whether various ER-localized SNARE proteins are involved in this fusion process. When phagosomes were isolated from murine J774 macrophages, we found that ER-localized SNARE proteins (syntaxin 18, D12, and Sec22b) were significantly enriched in the phagosomes. Fluorescence and immuno-EM analyses confirmed the localization of syntaxin 18 in the phagosomal membranes of J774 cells stably expressing this protein tagged to a GFP variant. To examine whether these SNARE proteins are required for phagocytosis, we generated 293T cells stably expressing the Fc receptor, in which phagocytosis occurs in an IgG-mediated manner. Expression in these cells of dominant-negative mutants of syntaxin 18 or D12 lacking the transmembrane domain, but not a Sec22b mutant, impaired phagocytosis. Syntaxin 18 small interfering RNA (siRNA) selectively decreased the efficiency of phagocytosis, and the rate of phagocytosis was markedly enhanced by stable overexpression of syntaxin 18 in J774 cells. Therefore, we conclude that syntaxin 18 is involved in ER-mediated phagocytosis, presumably by regulating the specific and direct fusion of the ER and plasma or phagosomal membranes.

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E05-12-1174) on June 21, 2006.

Address correspondence to: Kiyotaka Hatsuzawa (hatsu{at}fmu.ac.jp)

Abbreviations used: ER, endoplasmic reticulum; SNARE, soluble N-ethylmaleimide-sensitive factor attachment protein receptor; ECL, enhanced chemiluminescence; COP, coat protein; TMD, transmembrane domain; VSVG-EGFP, enhanced green fluorescent protein-tagged glycoprotein of vesicular stomatitis virus ts045.

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