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 ) 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 α/β–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 , 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 , 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 ).
Fascicles are clearly important for neuron growth—axons that leave fascicles travel three times slower—but 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.
By William A. Wells, 1095 Market St. #516, San Francisco, CA 94103. E-mail: firstname.lastname@example.org