The conserved YAGL motif in human metapneumovirus is required for higher-order cellular assemblies of the matrix protein and for virion production.

YXXL motifs in cellular and viral proteins have a variety of functions. The matrix (M) protein of the respiratory pathogen human metapneumovirus (hMPV) contains two such conserved motifs--YSKL and YAGL. We mutated these sequences to analyze their contributions to hMPV infectivity. The mutant clones were capable of intracellular replication; however, the YAGL but not YSKL mutants were defective at spreading in infected cultures. We improved the reverse genetics system for hMPV and generated cell lines that stably expressed selectable, replicating full-length genomes for both the wild type and the mutant clones, allowing microscopic and biochemical analyses of these viruses. YAGL mutants produced normal cellular levels of M protein but failed to release virions, while ectopic coexpression of wild-type M generated particles that were restricted to a single cycle of infection. The YAGL motif did not act as a late (L) domain, however, since hMPV budding was independent of the cellular endosomal sorting complex required for transport (ESCRT) machinery and because replacement of the YAGL motif with classical L domains generated defective viruses. Instead, the YAGL mutants had defective M assemblies lacking a normal filamentous appearance and showed poor extractability from the cell compared to the wild-type protein. The mutant proteins were not grossly misfolded, however, as they interacted with cellular membranes and coassembled with wild-type M proteins. Thus, the YAGL motif is an important determinant of hMPV assembly. Furthermore, the selectable hMPV genomes described here should extend the use of reverse genetics systems in the analysis of spreading-defective viruses.