Biomedical Engineering Reference
In-Depth Information
In some systems, proximity of donor and acceptor organelle
alleviates this problem. For example
cis
-Golgi is present in close
proximity (about 150 nm) to ER exit sites in yeast,
C. elegans
, and
D. melanogaster
[
8
,
59
-
61
]. Given that an ER-derived COPII
vesicle is about 70 nm in diameter, targeting is easily achieved by
diffusion and subsequent engagement with tethering factors.
However, how long-range transport, in particular to the plasma
membrane is regulated remains poorly understood and predicted
regulatory factors are still missing (Beerepoot et al., Chapter
8
)
.
Proteomic and protein-protein interaction studies may greatly aid
the identifi cation and understanding the function of such regulators
(Weber et al., Chapter
20
;
Gheiratmand et al., Chapter
29
). In
most in vitro systems used to study intracellular transport, such
components may not be essential, and could therefore be missed.
It also remains unclear how SNARE proteins are included into
vesicles and how t-SNAREs get to their fi nal compartment without
causing fusion along the way. It has been shown that some SNARE
proteins can interact directly with the COPII component Sec24,
and SNARE proteins can interact at least in vitro with COPI com-
ponents in an ArfGAP-dependent manner [
62
,
63
]. These fi nding
let to the proposal that SNAREs might be the starting point upon
which to assemble coat proteins and initiate the formation of a
vesicle [
44
,
64
]. However, how SNAREs are distributed into post-
Golgi departments is not satisfactorily understood to date. More
sophisticated methods will have to be employed to shed light on
these important issues.
5
Future Directions
Most of what we know about protein translocation is deduced from
the study of a small number of model proteins that were analyzed
in a few model organisms. In order to generalize these fi ndings, it
will be very important to base our knowledge on a broader ground.
The advent of the postgenomic era now allows the analysis of mem-
brane traffi cking processes on genome- and proteome-wide scales.
For example, the use of yeast knockout collections offers a straight-
forward and cheap strategy to identify the full complement of com-
ponents required for a given process. A wonderful example for the
infl uence of such a screen is the study by the laboratory of Bill
Wickner, which identifi ed in one study137 genes that are needed
for normal vacuole size and copy number in growing cells [
65
].
Similarly, siRNA libraries to knockdown individual proteins in tis-
sue culture cells have increased the repertoire or regulatory factors
known to date. The use of such yeast knockout arrays and reverse
genetic methods is described in several chapters in this topic (Dalton
et al., Chapter
27
; Bard et al., Chapter
28
; Gheiratmand, Chapter
29
;
Lipovsky et al., Chapter
30
;
Gaziova et al., Chapter
26
)
.
