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A conserved alpha-herpesvirus protein necessary for axonal localization of viral membrane proteins.

Tomishima MJ, Enquist LW - J. Cell Biol. (2001)

Bottom Line: We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins.The data support a model where virion subassemblies but not complete virions are transported in the axon.Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA.

ABSTRACT
Pseudorabies virus, an alpha-herpesvirus, is capable of infecting the nervous system and spreading between synaptically connected neurons in diverse hosts. At least three viral membrane proteins (gE, gI, and Us9) are necessary for the spread of infection from presynaptic to postsynaptic neurons (anterograde spread) in infected rodents. To understand how these proteins effect anterograde spread between neurons, we analyzed the subcellular localization of viral proteins after infection of cultured rat sympathetic neurons with wild-type or mutant viruses. After Us9- mutant infections but not gE- mutant infections, only a subset of the viral structural proteins had entered axons. Surprisingly, capsid and tegument proteins but not viral membrane proteins were detected in axons. The spread of Us9 missense mutants in the rodent nervous system correlated with the amount of viral membrane proteins localized to axons. We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins. The data support a model where virion subassemblies but not complete virions are transported in the axon. Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

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A model for Us9-dependent axonal sorting of viral membrane proteins. Us9 is depicted as a white rectangle, and other viral membrane proteins are black “lollipops.” The adaptor complex AP-1 is shown as a hexagon, and AP-3 is shown as a star. (A) The targeting of viral membrane proteins to the axon requires Us9. A tyrosine-based cytoplasmic sorting signal in Us9 interacts with AP-3, allowing the formation of axonal transport vesicles coated with this adaptor complex. Other viral membrane proteins are passive “cargo” that are targeted to the axon due to an association with Us9, not a cis-acting peptide sequence. The cytoplasmic tails of other viral membrane proteins can still interact with other adaptor complexes in wild-type infections, allowing transport vesicles to traffic to other regions of the neuron in addition to the axon. (B) Without Us9, no axonal-sorting signal is present on the cytoplasmic tails of viral membrane proteins. Vesicles still form but are no longer targeted to the axon. The cytoplasmic tails of other viral membrane proteins still interact with other adaptor complexes, allowing transport vesicles to traffic to other regions of the neuron.
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fig10: A model for Us9-dependent axonal sorting of viral membrane proteins. Us9 is depicted as a white rectangle, and other viral membrane proteins are black “lollipops.” The adaptor complex AP-1 is shown as a hexagon, and AP-3 is shown as a star. (A) The targeting of viral membrane proteins to the axon requires Us9. A tyrosine-based cytoplasmic sorting signal in Us9 interacts with AP-3, allowing the formation of axonal transport vesicles coated with this adaptor complex. Other viral membrane proteins are passive “cargo” that are targeted to the axon due to an association with Us9, not a cis-acting peptide sequence. The cytoplasmic tails of other viral membrane proteins can still interact with other adaptor complexes in wild-type infections, allowing transport vesicles to traffic to other regions of the neuron in addition to the axon. (B) Without Us9, no axonal-sorting signal is present on the cytoplasmic tails of viral membrane proteins. Vesicles still form but are no longer targeted to the axon. The cytoplasmic tails of other viral membrane proteins still interact with other adaptor complexes, allowing transport vesicles to traffic to other regions of the neuron.

Mentions: We propose a model (Fig. 10) where viral membrane proteins achieve a steady-state localization in the trans-Golgi network or a post-Golgi compartment in the cell body. Once in this compartment, viral membrane proteins are selectively incorporated into vesicles destined for the axon. In the simplest case, the presence of Us9 in the compartment recruits a neuron-specific adaptor complex that binds to Us9 via the dityrosine motif (for example, some forms of AP-3 are possible candidates) (Pevsner et al., 1994), leading to the formation of transport vesicles carrying Us9 and other viral membrane proteins “coated” with the axonal adaptors. Therefore, in the absence of Us9 vesicles, containing viral membrane proteins would not be directed to the axon. The cytoplasmic tails of other viral membrane proteins could still bind to other adaptor complexes, allowing vesiculation and transport to the cell surface or movement to compartments other than the axon. In this model, only one molecule in a transport vesicle need carry the cis-acting axonal sorting signal. Such a “piggyback” model of axonal targeting of neuronal proteins has been proposed recently (Roos and Kelly, 2000). Other proteins within the vesicle gain access to the axon simply by being in the same vesicle as the protein with the sorting signal. We propose that Us9 represents such an axon-specific sorting molecule and that other viral membrane proteins are merely “cargo.” However, our observation that not all of the vesicles within the axon contain Us9 is noteworthy. Perhaps only vesicles moving into the axon contain Us9, whereas the non-Us9–containing vesicles enriched in the distal axon represent viral membrane proteins that have accumulated within an organelle (for example, an endosome). Alternatively, Us9 may function in the cell body of the neuron to alter the trafficking patterns of viral membrane proteins. We are in the process of testing these ideas.


A conserved alpha-herpesvirus protein necessary for axonal localization of viral membrane proteins.

Tomishima MJ, Enquist LW - J. Cell Biol. (2001)

A model for Us9-dependent axonal sorting of viral membrane proteins. Us9 is depicted as a white rectangle, and other viral membrane proteins are black “lollipops.” The adaptor complex AP-1 is shown as a hexagon, and AP-3 is shown as a star. (A) The targeting of viral membrane proteins to the axon requires Us9. A tyrosine-based cytoplasmic sorting signal in Us9 interacts with AP-3, allowing the formation of axonal transport vesicles coated with this adaptor complex. Other viral membrane proteins are passive “cargo” that are targeted to the axon due to an association with Us9, not a cis-acting peptide sequence. The cytoplasmic tails of other viral membrane proteins can still interact with other adaptor complexes in wild-type infections, allowing transport vesicles to traffic to other regions of the neuron in addition to the axon. (B) Without Us9, no axonal-sorting signal is present on the cytoplasmic tails of viral membrane proteins. Vesicles still form but are no longer targeted to the axon. The cytoplasmic tails of other viral membrane proteins still interact with other adaptor complexes, allowing transport vesicles to traffic to other regions of the neuron.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2196449&req=5

fig10: A model for Us9-dependent axonal sorting of viral membrane proteins. Us9 is depicted as a white rectangle, and other viral membrane proteins are black “lollipops.” The adaptor complex AP-1 is shown as a hexagon, and AP-3 is shown as a star. (A) The targeting of viral membrane proteins to the axon requires Us9. A tyrosine-based cytoplasmic sorting signal in Us9 interacts with AP-3, allowing the formation of axonal transport vesicles coated with this adaptor complex. Other viral membrane proteins are passive “cargo” that are targeted to the axon due to an association with Us9, not a cis-acting peptide sequence. The cytoplasmic tails of other viral membrane proteins can still interact with other adaptor complexes in wild-type infections, allowing transport vesicles to traffic to other regions of the neuron in addition to the axon. (B) Without Us9, no axonal-sorting signal is present on the cytoplasmic tails of viral membrane proteins. Vesicles still form but are no longer targeted to the axon. The cytoplasmic tails of other viral membrane proteins still interact with other adaptor complexes, allowing transport vesicles to traffic to other regions of the neuron.
Mentions: We propose a model (Fig. 10) where viral membrane proteins achieve a steady-state localization in the trans-Golgi network or a post-Golgi compartment in the cell body. Once in this compartment, viral membrane proteins are selectively incorporated into vesicles destined for the axon. In the simplest case, the presence of Us9 in the compartment recruits a neuron-specific adaptor complex that binds to Us9 via the dityrosine motif (for example, some forms of AP-3 are possible candidates) (Pevsner et al., 1994), leading to the formation of transport vesicles carrying Us9 and other viral membrane proteins “coated” with the axonal adaptors. Therefore, in the absence of Us9 vesicles, containing viral membrane proteins would not be directed to the axon. The cytoplasmic tails of other viral membrane proteins could still bind to other adaptor complexes, allowing vesiculation and transport to the cell surface or movement to compartments other than the axon. In this model, only one molecule in a transport vesicle need carry the cis-acting axonal sorting signal. Such a “piggyback” model of axonal targeting of neuronal proteins has been proposed recently (Roos and Kelly, 2000). Other proteins within the vesicle gain access to the axon simply by being in the same vesicle as the protein with the sorting signal. We propose that Us9 represents such an axon-specific sorting molecule and that other viral membrane proteins are merely “cargo.” However, our observation that not all of the vesicles within the axon contain Us9 is noteworthy. Perhaps only vesicles moving into the axon contain Us9, whereas the non-Us9–containing vesicles enriched in the distal axon represent viral membrane proteins that have accumulated within an organelle (for example, an endosome). Alternatively, Us9 may function in the cell body of the neuron to alter the trafficking patterns of viral membrane proteins. We are in the process of testing these ideas.

Bottom Line: We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins.The data support a model where virion subassemblies but not complete virions are transported in the axon.Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA.

ABSTRACT
Pseudorabies virus, an alpha-herpesvirus, is capable of infecting the nervous system and spreading between synaptically connected neurons in diverse hosts. At least three viral membrane proteins (gE, gI, and Us9) are necessary for the spread of infection from presynaptic to postsynaptic neurons (anterograde spread) in infected rodents. To understand how these proteins effect anterograde spread between neurons, we analyzed the subcellular localization of viral proteins after infection of cultured rat sympathetic neurons with wild-type or mutant viruses. After Us9- mutant infections but not gE- mutant infections, only a subset of the viral structural proteins had entered axons. Surprisingly, capsid and tegument proteins but not viral membrane proteins were detected in axons. The spread of Us9 missense mutants in the rodent nervous system correlated with the amount of viral membrane proteins localized to axons. We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins. The data support a model where virion subassemblies but not complete virions are transported in the axon. Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

Show MeSH
Related in: MedlinePlus