Dictyostelium vaults: disruption of the major proteins reveals growth and morphological defects and uncovers a new associated protein.

Vaults are large cytoplasmic ribonucleoprotein particles that are highly conserved in both morphology and protein composition. Protein components of vaults isolated from Dictyostelium discoideum migrate on SDS-polyacrylamide gels as two bands, one at 94 kDa (MvpA) and the other at 92 kDa (MvpB). An MvpB cDNA clone was isolated from a Dictyostelium expression library. MvpB shares 60% identity with MvpA at the amino acid level. This cDNA has been used to disrupt the single mvpB gene in both wild-type and mvpA- genetic backgrounds. Although the mvp- mutant lines are viable, they show that loss of MvpA and/or MvpB interferes with vault function sufficiently to impede growth under conditions of nutritional stress. The resulting mutant cell lines reach stationary phase in suspension culture at one-third of the density of wild-type cells. Ovoid structures isolated from mvp- single mutant lines represent what remains of vaults in these cells. Similar ovoid structures isolated from the mvpA- mvpB- line copurify with a newly identified protein of 92 kDa (MvpC), which lacks cross-reactivity with currently available anti-vault antibodies. Our results indicate that the major vault proteins are necessary for optimal cell growth in Dictyostelium and reveal an unanticipated complexity in vault composition.