Human U1 snRNA forms a new chromatin-associated snRNP with TAF15

The U1 small nuclear RNA (snRNA)-in the form of the U1 spliceosomal Sm small nuclear ribonucleoprotein particle (snRNP) that contains seven Sm and three U1-specific RNP proteins-has a crucial function in the recognition and removal of pre-messenger RNA introns. Here, we show that a fraction of human U1 snRNA specifically associates with the nuclear RNA-binding protein TBP-associated factor 15 (TAF15). We show that none of the known protein components of the spliceosomal U1-Sm snRNP interacts with the newly identified U1-TAF15 snRNP. In addition, the U1-TAF15 snRNP tightly associates with chromatin in an RNA-dependent manner and accumulates in nucleolar caps upon transcriptional inhibition. The Sm-binding motif of U1 snRNA is essential for the biogenesis of both U1-Sm and U1-TAF15 snRNPs, suggesting that the U1-TAF15 particle is produced by remodelling of the U1-Sm snRNP. A demonstration that human U1 snRNA forms at least two structurally distinct snRNPs supports the idea that the U1 snRNA has many nuclear functions.