A doughnut-shaped heteromer of human Sm-like proteins binds to the 3'-end of U6 snRNA, thereby facilitating U4/U6 duplex formation in vitro

Abstract

We describe the isolation and molecular characterization of seven distinct proteins present in human [U4/U6·U5] tri‐snRNPs. These proteins exhibit clear homology to the Sm proteins and are thus denoted LSm (like Sm) proteins. Purified LSm proteins form a heteromer that is stable even in the absence of RNA and exhibits a doughnut shape under the electron microscope, with striking similarity to the Sm core RNP structure. The purified LSm heteromer binds specifically to U6 snRNA, requiring the 3′‐terminal U‐tract for complex formation. The 3′‐end of U6 snRNA was also co‐precipitated with LSm proteins after digestion of isolated tri‐snRNPs with RNaseT₁. Importantly, the LSm proteins did not bind to the U‐rich Sm sites of intact U1, U2, U4 or U5 snRNAs, indicating that they can only interact with a 3′‐terminal U‐tract. Finally, we show that the LSm proteins facilitate the formation of U4/U6 RNA duplices in vitro , suggesting that the LSm proteins may play a role in U4/U6 snRNP formation.