As Kedersha’s early analyses suggested, vaults are composed of multiple copies of at least four distinct components: three proteins and one RNA molecule. The major vault protein (MVP) accounts for some 75 percent of the particles’ mass, with each vault containing 78 copies of the protein. In fact, the expression of MVP in an insect cell line—insects themselves are one of the few eukaryotic organisms that don’t have vaults—results in the spontaneous formation of particles with morphologic characteristics similar to those of endogenous vaults.2 Another protein typically found in vaults is vault poly(ADP-ribose) polymerase (VPARP). VPARP and MVP mRNA transcripts are expressed in similar patterns in the cell, and subcellular fractionation studies point to a strong binding between the two proteins.[my emphasis]
The third vault protein is TEP1, previously identified as the mammalian telomerase-associated protein 1, which binds RNA in the telomerase complex. TEP1-knockout mice exhibited no alterations in telomerase function, suggesting its role in the nucleus is redundant, but vaults purified from these animals revealed a complete absence of the fourth component of vaults: vault RNA (vRNA), a small untranslated RNA found at the tips of the particles. This work pointed to TEP1’s role in the recruitment and stabilization of vRNA.
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