U1 snRNP-Dependent Suppression of Polyadenylation: Physiological Role and Therapeutic Opportunities in Cancer
Figure 1
Modes of U1 snRNP activity in splicing and suppression of 3′ end processing. (a) Role of U1 snRNP in splicing. The 5′ end of U1 snRNA base-pairs to the 5′ splice site cotranscriptionally, to define the functional splice donor site. The process is positively and negatively modulated by splicing factors binding to exonic and intronic splicing enhancer and silencers (ESE, ISE, ESS, and ISS, resp.). (b) Role of U1A protein in suppression of IgM cleavage and polyadenylation. U1 snRNP component U1A binds to multiple motifs (blue boxes) upstream or downstream of the intronic PAS (purple box) in IgM intron. These sites contain the A(U/G)GCN1−3C consensus motif and are similar to U1A-binding site on U1 snRNA. From the upstream sites, U1A multimers interact directly with the C-terminal domain of PAP through a binding pocket formed by the basic-residue motifs (green triangles), blocking polyadenylation. When U1A binds downstream, it prevents CstF binding to the G/U element (light purple box) and thus interferes with cleavage. (c) Role of U1 snRNP in suppression of BPV polyadenylation. U1 binds to a 5′ ss a few nucleotides upstream of the alternative PAS. Like U1A multimers, U1-70K interacts directly with the C-terminal domain of PAP through a binding pocket formed by the basic-residue motifs, blocking polyadenylation. (d) Tethered U1-70 K suppresses polyadenylation. A modified U1-MS2 snRNA is engineered to contain an MS2-binding loop in place of the U1-70 K binding loop. A U1-binding site represses polyadenylation and expression from a luciferase reporter vector when the wt U1 snRNA is expressed. The mutant U1-MS2 snRNA recapitulates PAP inhibition only when a fusion MS2-70 K protein is also coexpressed. (e) U1 adaptors recruit U1 snRNP to suppress polyadenylation. Single-stranded, bifunctional modified ASOs are used to recruit endogenous U1 to target sites within ~1 Kb region upstream of a PAS. The targeting moiety (red) base-pairs to a unique, transcript-specific sequence, while the recruiting moiety contains a consensus 5′ ss sequence to bind U1 snRNP with high affinity. The tethered U1 snRNP suppresses polyadenylation and the mRNA is then degraded by the 3′-5′ exosome. Large blue boxes represent exons/coding regions.
