Proposed model for ATG9A role in the establishment of liquid IAV inclusions.

We currently view liquid viral inclusions, composed of Rab11a endosomes and vRNPs, as sites dedicated to the assembly of the IAV genome [4,6,49]. We have previously shown that liquid viral inclusions develop in close contact with the ERES [4]. Here, we describe the initial events on the left panel that may lead to the formation of liquid viral inclusions on the right to facilitate the formation of IAV genomic complex. In this study, we demonstrate that IAV infection reduces the Rab11a-regulated recycling capacity of the host cell (step 1). This effect is likely a consequence of vRNP binding to Rab11a endosomes, which are then rerouted to the ERES to form viral inclusions. Such trafficking of Rab11a endosomes carrying the vRNPs to the ER is regulated by the host factor ATG9A. We identified that ATG9A is mobilized from the Golgi during IAV infection (step 2) and leads to the removal of Rab11a-vRNP complexes from microtubules when at the ER (step 3). It is thus possible that ATG9A moves to the ER to promote the linkage of viral inclusions to microtubules. In this location, vRNPs-Rab11a units may establish multiple and dynamic contacts forming liquid percolation-driven condensates. We also show (although with overexpression experiments) that ATG9A engages in multiple contacts with viral inclusions (step 4). We propose that the liquid properties of viral inclusions favor the formation of the 8-segmented IAV genome that is transported to the plasma membrane (step 5). ATG9A, autophagy related gene 9A; ER, endoplasmic reticulum; ERES, ER exit site; IAV, influenza A virus; vRNP, viral ribonucleoprotein.