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J. Cell Biol.,
Volume 143, Number 6, December 14, 1998 1485-1503
Cell Biology and Metabolism Branch, National Institutes of Health, National Institute of Child Health and Human
Development, Bethesda, Maryland 20892; Quantitative time-lapse imaging data of single cells expressing the transmembrane protein, vesicular stomatitis virus ts045 G protein fused to green fluorescent protein (VSVG-GFP), were used for kinetic
modeling of protein traffic through the various compartments of the secretory pathway. A series of first order rate laws was sufficient to accurately describe
VSVG-GFP transport, and provided compartment residence times and rate constants for transport into and
out of the Golgi complex and delivery to the plasma
membrane. For ER to Golgi transport the mean rate
constant (i.e., the fraction of VSVG-GFP moved per
unit of time) was 2.8% per min, for Golgi to plasma
membrane transport it was 3.0% per min, and for transport from the plasma membrane to a degradative site it
was 0.25% per min. Because these rate constants did
not change as the concentration of VSVG-GFP in different compartments went from high (early in the experiment) to low (late in the experiment), secretory
transport machinery was never saturated during the experiments.
The processes of budding, translocation, and fusion
of post-Golgi transport intermediates carrying VSVG-
GFP to the plasma membrane were also analyzed using
quantitative imaging techniques. Large pleiomorphic
tubular structures, rather than small vesicles, were
found to be the primary vehicles for Golgi to plasma
membrane transport of VSVG-GFP. These structures
budded as entire domains from the Golgi complex and
underwent dynamic shape changes as they moved along
microtubule tracks to the cell periphery. They carried
up to 10,000 VSVG-GFP molecules and had a mean
life time in COS cells of 3.8 min. In addition, they fused
with the plasma membrane without intersecting other
membrane transport pathways in the cell. These properties suggest that the post-Golgi intermediates represent a unique transport organelle for conveying large
quantities of protein cargo from the Golgi complex directly to the plasma membrane.
Center for Studies of Physics and Biology, Rockefeller University, New York,
New York 10021; and § BioInformatics Services, Rockville, Maryland 20854
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