Lecture 7, retrieval to ER with movie - Lecture 7 Resident...

Lecture 7
Resident ER proteins

What happens to polypeptides that are
transported into the ER?
1. Many become N-linked glycosylated
and undergo glucose trimming and
addition.
2. Disulfide bond formation on some.
3. Protein folding for all, and export of
misfolded proteins back into cytosol (so
called quality control of protein folding).

Traffic between ER and Golgi occurs in both
directions!
Page 766 Molecular Biology of the Cell (© Garland Science 2008)

COPII coated vesicles: ER --> cis-Golgi
COPI coated vesicles: cis-Golgi --> ER
COPII: anterograde transport
COPI: retrograde (retrieval) transport
Figure 13-23b Molecular Biology of the Cell (© Garland Science 2008)

Why do cells need retrograde
transport (Golgi --> ER)?????

Bip is a soluble molecular chaperone
(i.e., not membrane-bound) that helps
proteins fold in the ER lumen. Since it
functions in the ER lumen, it must reside
there.
PDI is another example of a soluble ER
resident protein.
How are such proteins retained in the ER
in the face of enormous flux from ER?

How do soluble ER resident
proteins remain in the ER?
1. Hypothesis: Signal sequence on soluble
resident ER proteins either acts to retain
proteins in the ER or acts to retrieve such
proteins from a later secretory compartment
back to the ER.