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Plenty of room at the inn
HEADLINES -
Plenty of room at the inn BARBARESE, E. et al. (1995) Protein translation components are colocalized in granules in oligodendrocytes 1. Cell Sci. 108, 2781-2790 A number of lines of evidence suggest that, in eukaryotic cells, many of the components of protein synthesis are organized structurally and spatially. Barbarese et al. have used detergentextracted murine oligodendrocytes in primary culture to visualize the intracellular distribution of a number of components of their protein translation machinery. By using highresolution fluorescence in situ hybridization and immunofluorescence in conjunction with dual-channel confocal laser scanning microscopy, the authors show that aminoacyl-tRNA synthetases, elongation factors, ribosomes and mRNAs all colocalize in granules in the processes, veins and membrane sheets of oligodendrocytes. This colocalization was evaluated by dual-channel cross-correlation analysis, and single-granule ratiometric analysis was used to determine the distribution of the different components in individual granules. The granules must be very large (radius of -0.7 pm), sufficient to house several hundred ribosomes. It is unlikely that the granules are artefacts: they have been observed by use of several different techniques, and may have gone unnoticed previously because conventional microscopy techniques are optimized for visualizing membranous intracellular organelles, and
452
NUDEL: the bare facts HONG, C. C. and HASHIMOTO, C. (1995) An unusual mosaic protein with a protease domain, encoded gene, is involved in defining embryonic dorsoventral polarity Cell 82, 785-794
by the nude/ in Drosophila.
Mutations in Drosophila genes affecting early embryonic polarity disrupt pathways extending from ovarian cells of the mother to embryonic nuclei. In the dorsoventral axis, genes responsible for initiating ventral development are activated by concentrating a transcriptional regulator, the DORSAL protein, into ventral nuclei of the syncytial embryo. Nuclear translocation of DORSAL requires activation of the TOLL receptor by its ligand, a product of proteolytic cleavage of the SPATZLE protein. This cleavage requires three genes encoding serineprotease-like molecules that constitute a zymogen activation cascade, which produces the ligand for TOLL ventrally. Since the TOLL receptor, SPATZLE and these zymogens are not located asymmetrically, how is activation of the ventral pathway restricted? Where lies the positional information that initiates polarity? This paper furthers the quest for upstream factors. The maternal activities of three genes (nude/, pipe and windbeutel) probably result in the asymmetric deposition of a ventralizing cue within the vitelline membrane surrounding the embryo. The first of these genes to be sequenced is the cDNAfor nude/. This is predicted to encode a large, secreted, modular protein with multiple glycosylation and glycosaminoglycan-addition sites, low-density lipoprotein (LDL) receptor repeats, an RCD sequence, a novel repeated sequence motif (WIID) and, most interestingly, two serine protease domains. Consistent with the mosaic nature of NUDEL, aspects of the nude1 phenotype (dorsalization and eggshell fragility) are uncoupled in a dorsalized point mutant that has normal eggshell. Furthermore, expression of NUDEL mRNA peaks in ventral ovarian follicle cells during formation of the vitelline membrane. The authors propose that NUDEL is both a structural component of the vitelline membrane matrix and also a scaffold, anchoring and activating locally the zymogen cascade. Is localization of NUDEL mRNA sufficient to initiate polarity or must NUDEL also be activated by proteolytic cleavage? The next step may involve determining whether NUDEL can either cleave itself or is cut by another protein to generate active fragments of immobilized product.
the granules are apparently not membrane-bound. It is proposed that these granules are supramolecular complexes containing all the necessary macromolecular components for protein translation and that they represent a subcellular
TRENDS
organization of the protein-translation machinery. It may be that this apparent spatial organization increases the efficiency of protein synthesis and also provides a vehicle for transport and localization of specific mRNAs within the cell.
IN CELL BIOLOGY
VOL.
5 DECEMBER
1995
Plenty of room at the inn BARBARESE, E. et al. (1995) Protein translation components are colocalized in granules in oligodendrocytes 1. Cell Sci. 108, 2781-2790 A number of lines of evidence suggest that, in eukaryotic cells, many of the components of protein synthesis are organized structurally and spatially. Barbarese et al. have used detergentextracted murine oligodendrocytes in primary culture to visualize the intracellular distribution of a number of components of their protein translation machinery. By using highresolution fluorescence in situ hybridization and immunofluorescence in conjunction with dual-channel confocal laser scanning microscopy, the authors show that aminoacyl-tRNA synthetases, elongation factors, ribosomes and mRNAs all colocalize in granules in the processes, veins and membrane sheets of oligodendrocytes. This colocalization was evaluated by dual-channel cross-correlation analysis, and single-granule ratiometric analysis was used to determine the distribution of the different components in individual granules. The granules must be very large (radius of -0.7 pm), sufficient to house several hundred ribosomes. It is unlikely that the granules are artefacts: they have been observed by use of several different techniques, and may have gone unnoticed previously because conventional microscopy techniques are optimized for visualizing membranous intracellular organelles, and
452
NUDEL: the bare facts HONG, C. C. and HASHIMOTO, C. (1995) An unusual mosaic protein with a protease domain, encoded gene, is involved in defining embryonic dorsoventral polarity Cell 82, 785-794
by the nude/ in Drosophila.
Mutations in Drosophila genes affecting early embryonic polarity disrupt pathways extending from ovarian cells of the mother to embryonic nuclei. In the dorsoventral axis, genes responsible for initiating ventral development are activated by concentrating a transcriptional regulator, the DORSAL protein, into ventral nuclei of the syncytial embryo. Nuclear translocation of DORSAL requires activation of the TOLL receptor by its ligand, a product of proteolytic cleavage of the SPATZLE protein. This cleavage requires three genes encoding serineprotease-like molecules that constitute a zymogen activation cascade, which produces the ligand for TOLL ventrally. Since the TOLL receptor, SPATZLE and these zymogens are not located asymmetrically, how is activation of the ventral pathway restricted? Where lies the positional information that initiates polarity? This paper furthers the quest for upstream factors. The maternal activities of three genes (nude/, pipe and windbeutel) probably result in the asymmetric deposition of a ventralizing cue within the vitelline membrane surrounding the embryo. The first of these genes to be sequenced is the cDNAfor nude/. This is predicted to encode a large, secreted, modular protein with multiple glycosylation and glycosaminoglycan-addition sites, low-density lipoprotein (LDL) receptor repeats, an RCD sequence, a novel repeated sequence motif (WIID) and, most interestingly, two serine protease domains. Consistent with the mosaic nature of NUDEL, aspects of the nude1 phenotype (dorsalization and eggshell fragility) are uncoupled in a dorsalized point mutant that has normal eggshell. Furthermore, expression of NUDEL mRNA peaks in ventral ovarian follicle cells during formation of the vitelline membrane. The authors propose that NUDEL is both a structural component of the vitelline membrane matrix and also a scaffold, anchoring and activating locally the zymogen cascade. Is localization of NUDEL mRNA sufficient to initiate polarity or must NUDEL also be activated by proteolytic cleavage? The next step may involve determining whether NUDEL can either cleave itself or is cut by another protein to generate active fragments of immobilized product.
the granules are apparently not membrane-bound. It is proposed that these granules are supramolecular complexes containing all the necessary macromolecular components for protein translation and that they represent a subcellular
TRENDS
organization of the protein-translation machinery. It may be that this apparent spatial organization increases the efficiency of protein synthesis and also provides a vehicle for transport and localization of specific mRNAs within the cell.
IN CELL BIOLOGY
VOL.
5 DECEMBER
1995