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© The Rockefeller University Press,
0021-9525/2000//1 $5.00
The Journal of Cell Biology, Volume 148, Number 3,
, 2000 1-2
In Brief |
© 2000 The Rockefeller University Press
Proteins on the Move Endosomes Get a Tail
Intracellular bacteria, such as Listeria, use actin tails to propel themselves through the cytoplasm. Clearly the bacteria have highjacked the cell's actin-polymerization machinery. But, based on the observations of Taunton et al. that cells make their own actin-rich tails Taunton et al. (page 519), Listeria appear to have hijacked the idea not just of polymerization, but also of tail formation. Taunton et al. characterize endosomal vesicles that move in Xenopus oocytes and extracts using such tails.
At least in extracts, the vesicles are identified as endosomes based on their multivesicular structure and staining with acridine orange. Endosomal membranes from HeLa cells move when added to extracts, whereas plasma and endoplasmic reticulum membranes do not. Purified Golgi membranes may yet show motile activity, as Cdc42 has been shown to have a role in Golgi trafficking.
A function for the tails remains purely speculative. Possibilities include an ATP-dependent diffusion mechanism for organelles and a method for nucleating actin away from the plasma membrane.
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The Role of Acidity in TGN Export |
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 Top The Role of Acidity... Telling Front from Back...β-Catenin Required for...Mitogenesis Versus Hypertrophy |
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Apical signal recognition is intact, as apical proteins are not redirected to the basolateral surface. And release of apical vesicles from the TGN is not altered, as the speed of apical delivery of a nonpolarized protein (human growth hormone) is not slowed. Thus the TGN machinery must be inhibited at the level of apical cargo incorporation into apical vesicles. Apical delivery may be mediated by incorporation of proteins into glycolipid-enriched rafts, so senior author Ora Weisz now plans to see if there are pH-dependent differences in the rafts before they are loaded into apical vesicles.
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Telling Front from Back Exu Gains an Unexpected Cargo |
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TopThe Role of Acidity...Telling Front from Back...β-Catenin Required for...Mitogenesis Versus Hypertrophy |
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Wilhelm et al. look for RNAs in the complex and find not bicoid (possibly because of an instability element in the 3' end of the bicoid RNA) but oskar. Exu and Yps proteins and oskar RNA share similar localization patterns, moving from the nurse cells to the anterior and then posterior of the oocyte. If, as expected, bicoid RNA is associated with the Exu complex, it must be shed before the complex moves to the posterior.
Flies lacking Exu show few or no defects in posterior development, so Exu RNA had not been previously suspected as an Exu cargo. Wilhelm et al. show, however, that accumulation of oskar RNA at the oocyte posterior is somewhat less efficient in flies lacking Exu. The remaining accumulation may occur through a combination of random mixing from cytoplasmic streaming and capture by anchoring proteins at the posterior.
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β-Catenin Required for Anterior Patterning |
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TopThe Role of Acidity...Telling Front from Back...β-Catenin Required for...Mitogenesis Versus Hypertrophy |
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Huelsken et al. created a β-catenin knockout mouse, as this molecule is a common effector of many Wnt signaling molecules. Before the mutant embryos die, several anterior differentiation markers are either not expressed, or their expression does not shift to the prospective anterior. Chimeras demonstrate that β-catenin is required in the fetus-forming epiblast, but not the visceral endoderm, suggesting that β-catenin mediates a signal from the embryonic ectoderm to pattern the endoderm. Early anterior patterning may also involve cell migration. The consequences of Wnt signaling for these processes, and the targets of β-catenin signaling, remain unknown.
The first set of β-catenin knockout mice, created by other investigators, was not examined for anterior gene expression. The mice did, however, show disruption of cadherin-mediated cell adhesion. This defect may have arisen from a shortened, dominant-negative protein made from the first, incomplete knockout. In the new knockout Huelsken et al. do not see a defect in adhesion, but observe that plakoglobin is recruited to replace the adhesive function of β-catenin.
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Mitogenesis Versus Hypertrophy |
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TopThe Role of Acidity...Telling Front from Back...β-Catenin Required for...Mitogenesis Versus Hypertrophy |
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Servant et al. have found the reason for these divergent responses Servant et al. (page 543). Both factors drive the accumulation of D-type cyclins and the activation of the partner Cdk4 kinase. But only PDGF-BB can fully activate the Cdk2 kinase needed for entry into S phase, because only PDGF-BB can convincingly turn off the transcription of the gene for p27Kip1, an inhibitor of Cdk2, and increase the turnover of the protein. The signal transduction pathways downstream of PDGF-BB and Ang II are at least partially overlapping, but analysis of the p27Kip1 promoter should help to determine the crucial differences.
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