Adeno-associated viruses (AAV) are non-enveloped parvoviruses widely utilized for therapeutic gene delivery. The membrane-associated accessory protein (MAAP), a key viral protein, is critical for cellular egress of both infectious and recombinant AAV particles. However, the structure-function correlates of this process have remained largely elusive. Here, we applied structure-guided evolution to enhance AAV egress and dissect the biology of both natural and synthetic MAAP (synMAAP) variants. Three structurally distinct combinatorial libraries focused on disordered domains within MAAP8 (derived from AAV serotype 8) were subjected to selection through both transfection and infectious cycling. Newly enriched synMAAPs obtained as a function of enhanced AAV secretion from host cells revealed a profound bias toward mutations within the C-terminal domain. The most potent synMAAPs showed increased propensity to form extended alpha-helix structures as predicted by structural modeling, amphipathicity, and hydrophobicity scores. Confocal microscopy demonstrated that, unlike natural MAAP8, synMAAPs are excluded from the nucleus and exhibit strong perinuclear accumulation within the Golgi. Proximity ligation analysis, biochemical analysis, and gene deletion studies further corroborated that synMAAPs appear to more efficiently engage SNARE complex proteins, notably involving VAMP3, to potentiate AAV egress. Together, these findings reveal new insights into structure-function correlates of viral egress from host cells and may help improve recombinant vector production for gene therapy applications.IMPORTANCEViruses must exit infected cells to spread, yet this process is poorly understood for adeno-associated virus. In this study, we examine how adeno-associated virus exits mammalian cells without causing cell death. By engineering and evolving new versions of the viral egress factor, we identify cellular trafficking pathways that are exploited during viral release. These findings improve our understanding of how viruses interact with host cells. Additionally, the engineered egress factors described in this study may be used to improve the efficiency of adeno-associated virus production for gene therapy applications.
Fusco et al. (Wed,) studied this question.