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I have read the latest issue (August 2016) of Nature Reviews
Molecular Cell Biology, and two of the articles on cell signaling are related
to some of the topics I was highly fascinated in when I was doing biochemistry
in my undergraduate years. The 2 review articles are about Rho family of GTPases and SNARE complexes. I
recommend these articles to those studying in biochemistry or molecular cell
biology in university.
The first article is about the regulation of Rho GTPases and
the proteins that regulate this particular family of G proteins [1]. Rho proteins are
important because it can mediate the changes in the structure of the actin
cytoskeleton, when it is subject to a number of upstream signals originated
from extercellular stimuli. When it is activated, the Rho protein can bind to a number of proteins
that have action on actin, such as ROCK, N-WASP, MRCK, WAVE 2, to name a few
examples. That will affect the G-Actin / F-Actin equilibrium, and affect the
assembly or disassembly of the actin cytoskeleton. The downstream signaling can
lead to effects such as cell migration, endosomal sorting, focal adhesion, and the formation of cytoskeletal structures
such as filopodia and lamellipodia.
I would illustrate with a quick example regarding actin
action at the endosome (Figure 1). When Cdc42, a Rho GTPase, is activated, it will in turn
activate 2 down stream proteins, N-WASP complex, and CIP4. N-WASP will in turn
activate the next down stream protein, Arp2/3 complex. N-WASP, CIP4, Arp2/3
will associate with each other as a multi-protein complex around the
internalizing endosome. F-actin will anchor itself at the protein complex, and
it will draw the internalization of the endosome, facilitating the endocytosis.
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| Figure 1. Example of Rho GTPase signalling. |
The review points out that, not only the obvious GDP-GTP recycling
can regulate the spatial-temporal action of the GTPase, there are other means
that the G Protein can be regulated. The formation of specific protein
complexes with the proteins associated with the action of GTPase (GEF, GAP,
GDI), and post-translational modification (PTM), which the latter is a hot
topic in chemical biology, are also possible means.
I would like to draw your attention to the fact that, as
stressed in the article, a phsophorylation on the Rho GTPase can have various
effects on the signaling. It can inactivate the GTPase through a GTP-GDP
recycling. It can mark the GTPase as a target for sequestration. It can also
direct it towards the action of E3 ligase and lead it to protein degradation
via the proteasome. Finally, it can lead to a subcellular localization, and in
some case (such as RhoU), the GTPase will be directed towards the endosomal
membrane, relevant for endocytic processes.
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| Figure 2. SNARE-mediated membrane fusion. |
The second article is about the SNARE complex (Figure 2), which in most
cases mediates intracellular membrane fusions [2]. The basic cycle is as
follows. The 2 membranes, which have SNARE proteins on their respective
surfaces as ‘contact points’, move towards each other. A docking event takes
place and the SNARE proteins associate with each other to lock up the 2
membranes. The pulling action increases the membrane curvature, and ‘breaks’ up
the inside by exposing the interior face of membrane. This leads to the contact
between the distal sides of the 2 membranes, forming a fusion pore as a result.
Finally, hen the curvature is relieved, the 2 membranes are fused together as a
new membrane. After that, the SNARE complexes disassemble and can be recycled,
via hydrolysis of ATP, to facilitate another round of membrane fusion. Though a
basic picture is known, we have to be cautious that some of the detailed points
are still controversial and demand further confirmation.
There are already a number of informative review articles
regarding the SNARE complex [3], yet this one has a slightly different focus. It
is found that, when the assembly of SNARE complex is carried out in vitro, it
is far slower in rate than the effective process that takes place in vivo. This
obviously suggests there are some missing components in the in vitro conditions
that hinder that from being a good model for the real process. Thus, a number
of techniques have been used to study and understand the action of the SNARE
complex in vivo, and they are reviewed in the article. For example, X-ray
crystallography has been used to study the complex formd from SNARE proteins
and other accessory proteins (Sec1-Munc18 protein for example), shedding light
on the mechanism. A particularly interesting X-ray structure is related to
neuronal SNARE complex – its binding to Synaptotagmin 1, the synaptic protein
that serves as a Ca2+ sensor and is involved in neuronal exocytosis
and regulation of release of neurotransmitter. On the other hand, cryo-electron
microscopy has also been employed to visualize the disassembly of SNARE complex
by NSF and SNAP proteins. Of course, better attempts for the re-constitution of
the cell systems in vitro has also led to a more realistic model for the in
vivo process.
I have also found a further paper which links up the 2
concepts together, and it should be very useful for us to see how all these
mechanisms (and other such as calcium and phosphoinositide signalling) working
together in a cellular system [4].
Hope you find all these useful!
by Ed Law
23/8/2016
Reference :
1. Regulating
Rho GTPases and their regulators, Richard G. Hodge and Anne J. Ridley,
Nature
Reviews Molecular Cell Biology, 2016, 17, 496–510. doi:10.1038/nrm.2016.67
2. Chaperoning
SNARE assembly and disassembly, Richard
W. Baker and Frederick M. Hughson
Nature
Reviews Molecular Cell Biology, 2016, 17, 465–479.
doi:10.1038/nrm.2016.65
3. For
example, see: (a) SNAREs — engines for membrane fusion, Reinhard Jahn and
Richard H. Scheller, Nature Reviews Molecular Cell Biology, 2006, 7,
631-643; (b) SNARE-mediated membrane fusion, Yu A. Chen and Richard H.
Scheller, Nature Reviews Molecular Cell Biology, 2001, 2, 98-106.
4. The
role of Rho GTPases and SNAREs in mediator release from granulocytes, Paige
Lacy, Pharmacology & Therapeutics, 2005, 107, 358 – 376.
