 |
Making Peroxisomes from (Almost) Nothing |
Peroxisomal deficiency diseases such as Zellweger syndrome would seem to be a promising place to start in defining how peroxisomes are made. But the cells of all Zellweger patients examined to date have contained
peroxisomes. The disease defects appear to be mainly in
the import of peroxisomal matrix proteins, an interesting process but not very helpful in determining how an organelle with no genome propagates itself. In this issue,
South and Gould (page 255) find a Zellweger cell line that
is mutant for the PEX16 gene and lacks detectable peroxisomal structures. Restoration of PEX16 expression causes
peroxisome regeneration.
How regeneration might work is mysterious. The Zellweger cells contain no membranous organelles with detectable amounts of any 1 of 11 different peroxisomal
membrane proteins, so a substrate for Pex16 action, a pre-peroxisomal vesicle, remains a hypothetical entity.
Peroxisome regeneration is slow, suggesting that most
peroxisome proliferation in normal cells is directed by
other means, such as Pex11 
/
-mediated vesiculation of
existing peroxisomes. This pathway still requires membrane insertion of newly synthesized integral membrane
proteins into peroxisomes, which may be the real (if unproven) function of Pex16.
| |
Multi-Step Nuclear Envelope Reassembly |
When the nuclear envelope is reassembled after mitosis,
the end result is inner and outer membranes that differ in
composition. There are two opposing models for this process. A uniform set of vesicles could dock onto chromatin
and then constituents of the two membranes could be separated from one another. Alternatively, fusion of distinguishable sets of vesicles, created by an ordered disassembly process, could create distinct membrane domains. On page 225, Drummond et al. report that Xenopus eggs have
at least two biochemically distinct sets of vesicles, which
dock onto chromatin in a defined order.
Drummond et al. use a published method to separate
the two vesicle populations, but they are the first to define
protein components that are unique to the two populations. NEP-B78, a protein that they identify using antibodies, is found only on the membrane pellet 2 (MP2) vesicle
population. This fraction is sufficient for vesicle binding to
sperm chromatin, but the vesicles do not fuse to form a nuclear envelope. MP1 vesicles can only bind to chromatin in
the presence of the MP2 fraction; after this event there is
fusion between the two vesicle types leading to the formation of a nuclear envelope. The MP1 fraction contains no
NEP-B78 but does contain a putative lamin B receptor, LBRx.
Consistent with these biochemical data, NEP-B78 attaches to chromatin before LBRx both in vitro and in vivo.
This is surprising, given that NEP-B78 ends up in the outer
membrane and LBRx in the inner membrane. NEP-B78
vesicles may be remodeling the chromatin to allow binding
of the LBRx vesicles.
| |
Rho as a Positional Signal for Cytokinesis |
On page 305, O'Connell et al. report that ectopic cleavage
furrows form in adherent cells in response to rho inhibition. They propose, therefore, that rho helps define the
timing and placement of the cleavage furrow in cytokinesis.
The new results are surprising, as rho inhibition in embryos prevents furrow formation, suggesting that rho promotes a localized contraction event to drive cytokinesis.
O'Connell et al. find that there is a similar failure of cytokinesis after rho inhibition in nonadherent HeLa cells. But
somehow adherent cells circumvent this rho function and
form furrows when rho is inhibited, without any accumulation of myosin or actin at the furrow. Perhaps in adherent
cells the two ends of the cell are, in effect, walking away
from each other to create the tension needed for furrow formation.
Without rho, the adherent cells form furrows that are
too numerous and too wide. This can be explained if rho
normally maintains the integrity of the actin cortex and
thus limits the region of weakness, the ingressing furrow.
Even in embryos, it has long been recognized that cortical
disassembly represents an important part of the cytokinesis process. When rho is inhibited, the whole cortex may
be weakened so that ectopic furrows form.
| |
Separating Guidance from Fasciculation |
The first landmark in the long journey of neurons from the
eye to the brain is the optic disk. The axons of retinal ganglion cells (RGCs) converge on this point; they then exit
the eye (with the help of the netrins) and distribute to
their correct topographic location in the brain's visual target centers (with the help of the ephrins).
Claudia Stuermer's group has shown that neurolin, a cell
adhesion molecule of the immunoglobulin superfamily, is
needed for RGCs to find the optic disk. This group now
finds that monoclonal antibodies specific to the first and
third immunoglobulin domains of neurolin inhibit the
bundling, or fasciculation, of RGCs, whereas monoclonals
against the second domain inhibit pathfinding (page 339).
Fascicles are clearly important for neuron growth
axons
that leave fascicles travel three times slowerbut growth
in a fascicle is insufficient for correct guidance. By video
microscopy, axons treated with the second-domain antibody (which has no effect in an in vitro fasciculation assay)
frequently deviate from the fascicle track. The loops and
spirals of RGC axons treated with neurolin antibodies suggest that repellent interactions may be involved. Determining whether neurolin detects an attractive cue, or turns off a repellent signal, will have to await identification of
the putative neurolin ligand.
| |
Disulfides in the Cytoplasm |
The redox potential of cellular compartments dictates that
proteins have free sulfhydryl groups in the cytoplasm, and
form disulfides in the extracellular fluid, endoplasmic reticulum, and Golgi. At least that is the received wisdom.
But on page 267, Krijnse Locker and Griffiths report that
certain vaccinia virus proteins have disulfide groups that
are exposed to the cytoplasm.
Vaccinia is a large and complex virus that assembles in
the cytoplasm before being wrapped by a double membrane derived from the intermediate compartment. As
with the nuclear membrane, the inner and outer vaccinia
membranes differ in their compositions. The wrapped particle, termed the intracellular mature virus (IMV), is infectious if the cell lyses, but most intercellular vaccinia transmission probably occurs only after another membrane
wrapping event, which creates the extracellular enveloped
virus (EEV).
Krijnse Locker and Griffiths find that three core proteins and three membrane proteins of the IMV have disulfide bonds. The core disulfides do not form if virus assembly is blocked, suggesting that the virus may create
the appropriate redox environment inside itself once the
membrane coating is sealed. However, disulfide formation
by the membrane proteins appears to be intrinsic to these
proteins, even though at least some of the disulfides are exposed to the cytoplasm. The authors suggest that the
disulfides form thanks to a combination of protein folding
and packing (to put the cysteines in close proximity) and
local redox effects in the protein.
Disulfide formation may help seal the IMVs, as inhibition of disulfide formation yields poorly infectious IMVs
in which the membranes open up to expose the core. Reduction of the core, but not membrane disulfides occurs
naturally after IMVs infect a cell. This process may aid in
virus uncoating, or in opening up the structure of the core
to allow viral transcription.
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