Functional Roles of the Novel Membrane-Associated AAV Protein MAAP

The adeno-associated virus (AAV) has evolved overlapping genes to maximize its genome use, as with the recently-discovered ORF in the cap gene which encodes a membrane-associated accessory protein (MAAP) located in the same genomic region as the VP1/2 unique domain. This 13 KDa protein, unique to the dependovirus genus, is not homologous to any known protein. To study the role of MAAP in wild-type AAV (wt-AAV), we made point mutations along the MAAP ORF while keeping overlapping capsid proteins' ORFs intact. In cells co-transfected with plasmids encoding wt-AAV genome and adenovirus helper genes, MAAP localized in the plasma membrane, as well as intracellular membranes. Both inactivation and truncation of MAAP translation affected the emergence and intracellular distribution of the AAV capsid proteins. Importantly, while MAAP was beneficial for wt-AAV replication/infection, it was not essential for AAV generation. Oppositely, some MAAP-modifications resulted in substantially improved virus yields and capsid integrity. Altogether, our results reveal some MAAP functions having important implications for better-quality and quantity production of AAV vectors for therapeutic purposes.