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Bos1p, an integral membrane protein of the endoplasmic reticulum to Golgi transport vesicles, is required for their competence
A transport-vesicle protein required for fusion LIAN, I. P. and FERRO-NOVICK,S. (1993) Bosl p, an integral membrane protein of the endoplasmic reticulum to Golgi transport vesicles, is required for their fusion competence Cell 73, 735-745 Although a number of soluble proteins involved in vesicle fusion have been identified in both yeast and higher eukaryotes, relatively little is known about the integral membrane proteins involved in this process. Previous studies in the FerroNovick lab have shown that the Saccharomyces cerevisiae integral membrane protein BOS1 is required for ER-to-Golgi transport, and that BOSl-depleted cells accumulate transport vesicles. Here, Lian and Ferro-Novick have used a permeabilized-cell assay to show that antiBOS1 antibodies specifically block
This month's headlineswere contributed by Catherine Brooksbank, Carolyn Elliss, Adrian Harwood, Stella Hurtley, BrycePaschaland Debbie Sweet.
256
ER-to-Golgi transport after the vesicle-budding step. Furthermore, they demonstrate that transport vesicles from cells lacking BOS1 cannot fuse with acceptor (Golgi) membranes, and that BOS1 is a constituent of ER-to-Golgi transport vesicles. Together these results suggest that the presence of BOS1 on transport vesicles derived from the ER is required for their fusion with the Golgi complex. This study is particularly pertinent in the light of recent results from the Rothman lab that suggest synaptic membrane proteins such as synapto-
brevin and syntaxin act as receptors on vesicle and target membranes for the NSF-SNAP 'fusogen' complex. It has been proposed that members of the synaptobrevin/syntaxin protein family, which include BOS1 and SEC22 (SEC22 is also present on BOSl-carrying vesicles and is required for ER-to-Golgi transport), are involved in various vesicle fusion events along the secretory pathway. Although there is as yet no evidence for an interaction between BOS1 and NSF-SI'.AP, one cannot help feeling that it may not be very far away.
A bridge to FAR
Kiss and run
PETER,M., GARTNER,A., HORECKA, l., AMMERER,G. and HERSKOWlTZ, I. (1993) FAR1 links the signal transduction pathway to the cell cycle machinery in yeast Cell 73, 747-760
AWAREZ DE TOLEDO, G., FERN~,NDEZ-CHACON,R. and FERNF.NDEZ,I. M. (1993) Releaseof secretory products during transient vesicle fusion Nature 363, 554-558
Much attention is currently focused on the link between growth-regu. latory signal transduction pathways initiated at the cell surface and the core machinery that controls the cell cycle. In Saccharomyces cerevisiae, the mating pheromone c~-factor arrests a cells in G1 phase, and induces transcription of mating genes. Genetic approaches have uncovered a number of proteins of the pathway activated by o~-factor, from a receptor coupled to a heterotrimeric G protein to a group of serine/ threonine kinases that includes the MAP-kinase family member FUS3. This paper now links the pathway to proteins directly involved in cell cycle control. The FAR1 protein is required for pheromone-induced cell cycle arrest and has been implicated in antagonizing Gl-cyclin function, but its molecular activity has been unclear. Peter et al. here provide evidence for a physical association of FAR1 and CDC28-CLN2, the kinase-cyclin complex that is required for the G1-S phase transition. Furthermore,
It has been known for some time that when secretory granules contact the plasr.~a membrane in exocytosis, the initial fusion pore that forms to link granule contents to the outside of the cell may close again (reversible or 'flickering' fusion), rather than enlarging to allow complete fusion. Are some vesicle contents released during the reversible membrane associations, avoiding the need for the membrane recycling that follows complete fusion? By combining measurements of membrane capacitance (area) with the technique of amperometry (which can detect released substances electrochemically) Alvarez de Toledo et al. show that this is indeed the case for serotonin release by giant granules of beige-mouse mast cells: a 'foot' in the trace of the amperometric signal correlates with times of flickering fusion (detected both as a prelude to full fusion and as an independent event). The authors suggest that this represents diffusion through the fusion pore of free serotonin, i.e. serotonin that is not bound to the matrix of the granule. From measurements of kinetics and amounts of release for granules of different sizes, the authors make the interesting prediction that small synaptic vesicles without a matrix core may release much of their content during flickering fusion.
FAR1 is shown to be a target of the FUS3 kinase when the pheromone signal transduction pathway is activated, and this phosphorylation of FAR1 is required for its efficient binding to CDC28-CLN2. In addition, studies with mutants of FAR1 demonstrate that the ability of FAR1 to bind CDC28-CLN2 and to arrest the
cell cycle correlate. The work answers much, but, as always, more questions arise. How does the association of FAR1 with CDC28-CLN2 control CDC28-CLN2 activity? Does FAR1 inhibit other G1 cyclins? Will the FAR1 story become a paradigm for the molecular basis of inhibition of cell growth?
TRENDS IN CELLBIOLOGYVOL. 3 AUGUST 1993
This month's headlineswere contributed by Catherine Brooksbank, Carolyn Elliss, Adrian Harwood, Stella Hurtley, BrycePaschaland Debbie Sweet.
256
ER-to-Golgi transport after the vesicle-budding step. Furthermore, they demonstrate that transport vesicles from cells lacking BOS1 cannot fuse with acceptor (Golgi) membranes, and that BOS1 is a constituent of ER-to-Golgi transport vesicles. Together these results suggest that the presence of BOS1 on transport vesicles derived from the ER is required for their fusion with the Golgi complex. This study is particularly pertinent in the light of recent results from the Rothman lab that suggest synaptic membrane proteins such as synapto-
brevin and syntaxin act as receptors on vesicle and target membranes for the NSF-SNAP 'fusogen' complex. It has been proposed that members of the synaptobrevin/syntaxin protein family, which include BOS1 and SEC22 (SEC22 is also present on BOSl-carrying vesicles and is required for ER-to-Golgi transport), are involved in various vesicle fusion events along the secretory pathway. Although there is as yet no evidence for an interaction between BOS1 and NSF-SI'.AP, one cannot help feeling that it may not be very far away.
A bridge to FAR
Kiss and run
PETER,M., GARTNER,A., HORECKA, l., AMMERER,G. and HERSKOWlTZ, I. (1993) FAR1 links the signal transduction pathway to the cell cycle machinery in yeast Cell 73, 747-760
AWAREZ DE TOLEDO, G., FERN~,NDEZ-CHACON,R. and FERNF.NDEZ,I. M. (1993) Releaseof secretory products during transient vesicle fusion Nature 363, 554-558
Much attention is currently focused on the link between growth-regu. latory signal transduction pathways initiated at the cell surface and the core machinery that controls the cell cycle. In Saccharomyces cerevisiae, the mating pheromone c~-factor arrests a cells in G1 phase, and induces transcription of mating genes. Genetic approaches have uncovered a number of proteins of the pathway activated by o~-factor, from a receptor coupled to a heterotrimeric G protein to a group of serine/ threonine kinases that includes the MAP-kinase family member FUS3. This paper now links the pathway to proteins directly involved in cell cycle control. The FAR1 protein is required for pheromone-induced cell cycle arrest and has been implicated in antagonizing Gl-cyclin function, but its molecular activity has been unclear. Peter et al. here provide evidence for a physical association of FAR1 and CDC28-CLN2, the kinase-cyclin complex that is required for the G1-S phase transition. Furthermore,
It has been known for some time that when secretory granules contact the plasr.~a membrane in exocytosis, the initial fusion pore that forms to link granule contents to the outside of the cell may close again (reversible or 'flickering' fusion), rather than enlarging to allow complete fusion. Are some vesicle contents released during the reversible membrane associations, avoiding the need for the membrane recycling that follows complete fusion? By combining measurements of membrane capacitance (area) with the technique of amperometry (which can detect released substances electrochemically) Alvarez de Toledo et al. show that this is indeed the case for serotonin release by giant granules of beige-mouse mast cells: a 'foot' in the trace of the amperometric signal correlates with times of flickering fusion (detected both as a prelude to full fusion and as an independent event). The authors suggest that this represents diffusion through the fusion pore of free serotonin, i.e. serotonin that is not bound to the matrix of the granule. From measurements of kinetics and amounts of release for granules of different sizes, the authors make the interesting prediction that small synaptic vesicles without a matrix core may release much of their content during flickering fusion.
FAR1 is shown to be a target of the FUS3 kinase when the pheromone signal transduction pathway is activated, and this phosphorylation of FAR1 is required for its efficient binding to CDC28-CLN2. In addition, studies with mutants of FAR1 demonstrate that the ability of FAR1 to bind CDC28-CLN2 and to arrest the
cell cycle correlate. The work answers much, but, as always, more questions arise. How does the association of FAR1 with CDC28-CLN2 control CDC28-CLN2 activity? Does FAR1 inhibit other G1 cyclins? Will the FAR1 story become a paradigm for the molecular basis of inhibition of cell growth?
TRENDS IN CELLBIOLOGYVOL. 3 AUGUST 1993