The yeast NMD3 gene was identified in
a two-hybrid screen using the nonsense-mediated mRNA decay
factor, Upf1p, as bait. NMD3 was shown to encode
an essential, highly conserved protein that associated
principally with free 60S ribosomal subunits. Overexpression
of a truncated form of Nmd3p, lacking 100 C-terminal amino
acids and most of its Upf1p-interacting domain, had dominant-negative
effects on both cell growth and protein synthesis and promoted
the formation of polyribosome half-mers. These effects
were eliminated by truncation of an additional 100 amino
acids from Nmd3p. Overexpression of the nmd3Δ100
allele also led to increased synthesis and destabilization
of some ribosomal protein mRNAs, and increased synthesis
and altered processing of 35S pre-rRNA. Our data suggest
that Nmd3p has a role in the formation, function, or maintenance
of the 60S ribosomal subunit and may provide a link for
Upf1p to 80S monosomes.