Periodic Reporting for period 1 - AvINFLUENZA (Molecular basis of avian influenza polymerase adaptation to human hosts)
Periodo di rendicontazione: 2019-07-01 al 2021-06-30
Influenza A virus (IAV) is responsible for 3–5 million severe cases every year, resulting in 250 000-500,000 deaths. Most influenza strains evolve exclusively in water birds, but some highly pathogenic avian strains (e.g. H5N1, H5N8 and H7N9) can infect humans with lethal consequences (up to 60% mortality) and are potential pandemic threats for humanity if they acquire the ability to transmit from human to human. However, these avian viruses need to acquire mutations in their viral polymerase, the molecule responsible for producing more copies of their genome, in order to adapt and therefore efficiently replicate in humans.
Few mutations are required for avian to human adaptation, and in particular mutation of a negative for a positively charged residue (glutamic for lysine) in the position “627” of the polymerase rescues viral replication in human cells. In fact, this mutation was present in the influenza viruses that produced the pandemics of 1918, 1957 and 1968. A few years ago was discovered that this adaptation occurs because there is difference between the avian and the human version of a protein called ANP32a, since the avian protein is 33 residues longer.
However, there is no information regarding how the viral polymerase interacts with ANP32a or how does mutation 627 helps an avian virus to adapt to human ANP32a.
Few mutations are required for avian to human adaptation, and in particular mutation of a negative for a positively charged residue (glutamic for lysine) in the position “627” of the polymerase rescues viral replication in human cells. In fact, this mutation was present in the influenza viruses that produced the pandemics of 1918, 1957 and 1968. A few years ago was discovered that this adaptation occurs because there is difference between the avian and the human version of a protein called ANP32a, since the avian protein is 33 residues longer.
However, there is no information regarding how the viral polymerase interacts with ANP32a or how does mutation 627 helps an avian virus to adapt to human ANP32a.
In this project we characterized the interaction of avian and human ANP32A with the viral polymerase carrying or not the 627 mutation. Although the complexes are both found to be highly dynamic, they exhibit significant differences.
In the complex involving the human-adapted proteins, polyvalent combinations of transient interactions stabilise the complex between a highly negatively charged region of ANP32a and a positively charged surface of the polymerase formed among other residues by 627. This polyvalency is less efficient in the avian interaction, due to the interruption of the basic surface in the presence of the negatively charged 627 residue. This weaker interaction is however compensated by the recruitment of additional sequences on the longer avian ANP32a. Interestingly, we found that the interaction of human ANP32a with human-adapted polymerase also implicates residues such as 590 and 591, which were reported to be mutated instead of residue 627 in the virus that produced the 2009 influenza pandemic.
Notably, the cross-interaction between human ANP32A and the avian-adapted polymerase exhibits neither of these possible stabilisation mechanisms, which may be related to the inability of avian influenza virus polymerase to function in human cells without mutation in the 627 position.
Using the experimental data that we acquired, we were able to propose a structural model of the complex formed by the polymerase and ANP32a. Interestingly, the cylindrical pocket where ANP32a interacts with the polymerase lies in the vicinity of other reported adaptive mutations such as 355 and 521, suggesting that the importance of these mutations is also related to the interaction with ANP32a.
In the complex involving the human-adapted proteins, polyvalent combinations of transient interactions stabilise the complex between a highly negatively charged region of ANP32a and a positively charged surface of the polymerase formed among other residues by 627. This polyvalency is less efficient in the avian interaction, due to the interruption of the basic surface in the presence of the negatively charged 627 residue. This weaker interaction is however compensated by the recruitment of additional sequences on the longer avian ANP32a. Interestingly, we found that the interaction of human ANP32a with human-adapted polymerase also implicates residues such as 590 and 591, which were reported to be mutated instead of residue 627 in the virus that produced the 2009 influenza pandemic.
Notably, the cross-interaction between human ANP32A and the avian-adapted polymerase exhibits neither of these possible stabilisation mechanisms, which may be related to the inability of avian influenza virus polymerase to function in human cells without mutation in the 627 position.
Using the experimental data that we acquired, we were able to propose a structural model of the complex formed by the polymerase and ANP32a. Interestingly, the cylindrical pocket where ANP32a interacts with the polymerase lies in the vicinity of other reported adaptive mutations such as 355 and 521, suggesting that the importance of these mutations is also related to the interaction with ANP32a.
In this study, we described and compared the molecular complexes formed by the human-adapted or avian-adapted influenza polymerase with the respective ANP32a host proteins. This was done in order to understand the nature of these interactions as well as the role that they play in the formation of viruses with pandemic potential.
Our analysis revealed the existence of a highly dynamic molecular interaction that exhibits very different binding modes with ANP32A. The information produced by our research allow us to speculate further on its role in viral function. For instance, it raises the possibility that the interaction with ANP32A is associated with conformational changes that have been observed in the polymerase during the viral cycle.
Recent observations have established that dimerisation of influenza polymerase is essential for the initiation of genomic RNA synthesis during replication. Moreover, it has been suggested that ANP32A plays a role in assembly or regulation of this dimerisation process, for example, by recruitment of a second polymerase to a replicating polymerase to initiate formation of the viral progeny. In this context, the polyvalent nature of the interaction between ANP32A and the polymerase may be of functional relevance, allowing for more than one polymerase to simultaneously bind to ANP32A thereby co-localizing two polymerases to facilitate viral replication.
In summary, the description of these highly dynamic species-specific assemblies reveals unique mechanistic insight into the role of the ANP32 family in host adaptation of avian influenza polymerase to human cells, informing the identification of novel targets for influenza virus inhibition.
Our analysis revealed the existence of a highly dynamic molecular interaction that exhibits very different binding modes with ANP32A. The information produced by our research allow us to speculate further on its role in viral function. For instance, it raises the possibility that the interaction with ANP32A is associated with conformational changes that have been observed in the polymerase during the viral cycle.
Recent observations have established that dimerisation of influenza polymerase is essential for the initiation of genomic RNA synthesis during replication. Moreover, it has been suggested that ANP32A plays a role in assembly or regulation of this dimerisation process, for example, by recruitment of a second polymerase to a replicating polymerase to initiate formation of the viral progeny. In this context, the polyvalent nature of the interaction between ANP32A and the polymerase may be of functional relevance, allowing for more than one polymerase to simultaneously bind to ANP32A thereby co-localizing two polymerases to facilitate viral replication.
In summary, the description of these highly dynamic species-specific assemblies reveals unique mechanistic insight into the role of the ANP32 family in host adaptation of avian influenza polymerase to human cells, informing the identification of novel targets for influenza virus inhibition.