Essential Cell Biology 5th edition

Secretory Pathways
521
ER
Golgi
apparatus
secretory
vesicles
protein destined
for secretion
normal cell
secretory mutant A
secretory mutant B
secretory mutant C
protein secreted
protein accumulates
in ER
protein accumulates
in Golgi apparatus
protein accumulates
in transport vesicles
Figure 15−28 Temperature-sensitive mutants have been used to dissect the protein secretory pathway in yeast. Mutations
in genes involved at different stages of the transport process, as indicated by the red X, result in the accumulation of proteins in the
ER, the Golgi apparatus, or transport vesicles.
Such GFP fusion proteins are widely used to study ECB5 the e15.28/15.28 protein can be used to measure the kinetics of its movement
through the secretory pathway. Movies 15.1, 15.9,
location and movement of proteins in cells (Figure
15−29). GFP fused to a protein that shuttles in and out of 15.10, and 15.13 demonstrate the power and beauty of
the nucleus, for example, can be used to study nuclear this technique.
transport events. GFP fused to a plasma membrane
(A)
(B)
(C)
(D)
Figure 15−29 Tagging a protein with GFP allows the resulting fusion protein to be tracked throughout the cell. In this
experiment, GFP is fused to a viral coat protein and expressed in cultured animal cells. In an infected cell, the viral protein moves
through the secretory pathway from the ER to the cell surface, where the virus particles are assembled. Red arrows indicate the
direction of protein movement. The viral coat protein used in this experiment contains a mutation that allows export from the ER only at
a low temperature. (A) At high temperatures, the fusion protein labels the ER. (B) As the temperature is lowered, the GFP fusion protein
rapidly accumulates at ER exit sites. (C) The fusion protein then moves to the Golgi apparatus. (D) Finally, the fusion protein is delivered
to the plasma membrane, shown here in a more close-up view. The halo between the two white arrowheads marks the spot where a
single vesicle has fused, allowing the fusion protein to incorporate ECB5 e15.29/15.29
into the plasma membrane. These images are stills taken from
Movie 15.9. (A–D, courtesy of Jennifer Lippincott-Schwartz.)