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J. Cell Biol.,
Volume 144, Number 5, March 8, 1999 869-881
* Howard Hughes Medical Institute, Department of Molecular and Cellular Physiology, Stanford University School of Medicine,
Stanford, California 94305-5428; and The ER/Golgi soluble NSF attachment protein receptor (SNARE) membrin, rsec22b, and rbet1
are enriched in ~1-µm cytoplasmic structures that lie
very close to the ER. These appear to be ER exit sites
since secretory cargo concentrates in and exits from these structures. rsec22b and rbet1 fused to fluorescent
proteins are enriched at ~1-µm ER exit sites that remained more or less stationary, but periodically emitted
streaks of fluorescence that traveled generally in the
direction of the Golgi complex. These exit sites were
reused and subsequent tubules or streams of vesicles followed similar trajectories. Fluorescent membrin-
enriched ~1-µm peripheral structures were more mobile and appeared to translocate through the cytoplasm
back and forth, between the periphery and the Golgi
area. These mobile structures could serve to collect secretory cargo by fusing with ER-derived vesicles
and ferrying the cargo to the Golgi. The post-Golgi
SNAREs, syntaxin 6 and syntaxin 13, when fused to fluorescent proteins each displayed characteristic patterns
of movement. However, syntaxin 13 was the only
SNARE whose life cycle appeared to involve interactions with the plasma membrane. These studies reveal
the in vivo spatiotemporal dynamics of SNARE proteins and provide new insight into their roles in membrane trafficking.
Medical School, Institute for Biomembranes, University of Utrecht, 3584CX Utrecht, The
Netherlands
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