Biomedical Engineering Reference
In-Depth Information
the ER in just 10-15 minutes. If we assume that one regulator, such as ARF,
co-ordinately holds up membrane flow and the progress of a specific set of
glycosylation enzymes, the result will be compartments with characteristic
markers. The degree to which each aspect of the block (to membranes flux
vs. glycosylation enzyme cycling) is tight will determine how spread out or
restricted the distribution of a given enzyme will be. This idea nicely explains
why the distribution of the same enzyme differs in different cell types: the
precise nature of the coregulation of membrane flow and glycosylation
enzyme cycling will depend on the cell type (which itself would depend on
the activity of that particular cell and its secretory needs). It would seem rea-
sonable in the above model to keep the glycosylation enzymes inactive
during their return cycle. I postulate that ARF controls this aspect as well by
inhibiting the activity of glycosylation enzymes during their cycle and only
activating them at a precise point. That ARF is in fact responsible for such a
process is suggested by the fact that ARF inactivation leads to inappropri-
ate activation of glycosylation enzymes (i.e. in the ER in BFA-treated cells).
Yeast
gea
mutants (defective in the Gea1p and Gea2p ARF GEFs) as well
as
arf
mutants have a similar phenotype (Anne Peyroche, Alain Rambourg
and C.L.J., unpublished data). The improper regulation of glycosylation
enzyme activation and inhibition could explain why these mutants have
severe glycosylation defects. Another important feature to incorporate into
this model is the co-ordination of membrane flux with anterograde cargo
movement. This process is likely to involve cargo receptors that guide or
escort proteins through the secretory pathway (Herrmann
et al.,
1999). At
the level of the ER, the COPII coat is clearly essential for this process, in
conjunction with the ARF-like GTPase Sar1p. Whether or not ARF is
involved in this process as well is an open question. ARF has a clear role in
the function of the COPI coat, which is important for sorting of glycosyla-
tion enzymes and transport machinery away from anterograde cargo in post-
ER compartments (Gaynor
et al.,
1998b). It is possible that ARF co-regulates
glycoslyation enzyme activity with anterograde cargo movement through
interaction or regulation of cargo receptors. This regulation might be
achieved either through direct interaction with cargo receptors or through
ARF-COP1 interactions. The latter possibility is supported by the fact that
COPI has been shown to have cargo-specific effects on ER-Golgi transport,
and, like
arf
and
gea
mutants,
copl
mutants have defects in glycosylation of
cargo proteins.
A fundamental issue in the regulated forward membrane flux model is
defining in molecular terms the force generating forward membrane flow.
The endoplasmic reticulum is the site of synthesis of the bulk of intracel-
lular lipids, and it has been estimated that it takes only four minutes for half
of the lipid content of the ER to be transported to the Golgi in mammalian
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