The M-r 193,000 vault protein is up-regulated in multidrug-resistant cancer cell lines

Vaults are 13 megadalton ribonucleoprotein particles composed largely of th e major vault protein (MW) and two high molecular weight proteins, p240 and p193, and a small vault RNA (vRNA). Increased levels of MVP expression, va ult-associated vRNA, and vaults have been linked directly to multidrug resi stance (MDR). To further define the putative role of vaults in MDR, we prod uced monoclonal antibodies against the M-r 193,000 vault protein and studie d its expression levels in various multidrug-resistant cell lines. We find that, like MW, p193 mRNA and protein levels are increased in various multid rug-resistant cell lines. Subcellular fractionation of vault particles reve aled that vault-associated p193 levels are increased in multidrug-resistant cells as compared with the parental, drug-sensitive cells. Furthermore, pr otein analysis of postnuclear supernatants and coimmunoprecipitation studie s show that drug-sensitive MVP-transfected tumor cells lack this up-regulat ion in vault-associated p193. Our observations indicate that vault formatio n is limited not only by the expression of the MVP but also by the expressi on or assembly of at least one of the other vault proteins.