Molecular Architecture of the Human Pre-mRNA 3′ Processing Complex

Pre-mRNA 3' end formation is an essential step in eukaryotic gene expression. Over half of human genes produce alternatively polyadenylated mRNAs, suggesting that regulated polyadenylation is an important mechanism for posttranscriptional gene control. Although a number of mammalian mRNA 3' processing factors have been identified, the full protein composition of the 3' processing machinery has not been determined, and its structure is unknown. Here we report the purification and subsequent proteomic and structural characterization of human mRNA 3' processing complexes. Remarkably, the purified 3' processing complex contains similar to 85 proteins, including known and new core 3' processing factors and over 50 proteins that may mediate crosstalk with other processes. Electron microscopic analyses show that the core 3' processing complex has a distinct kidney shape and is similar to 250 angstrom in length. Together, our data has revealed the complexity and molecular architecture of the pre-mRNA 3' processing complex.