SNU13

SNU13 (15.5K/Snu13p) is the universal nucleation factor that initiates hierarchical assembly of K-turn-containing ribonucleoprotein complexes, including the U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNA RNP (PMID:12417735, PMID:12810916). It recognizes the K-turn motif through an induced-fit mechanism: the free cognate RNA (e.g. the U4 5' stem-loop) is conformationally flexible and lacks stable structure, and 15.5K binding folds and stabilizes the kinked conformation (PMID:15659359). Binding of 15.5K is an obligatory first step that licenses subsequent recruitment of complex-specific partners — NOP56, NOP58, fibrillarin, TIP48, and TIP49 for the box C/D snoRNP, with stem II of the box C/D motif required for these downstream associations but not for 15.5K binding itself (PMID:12417735). For the U3 snoRNP, 15.5K binds two independent K-turns (the B/C and C'/D motifs), and its prior binding to the box B/C motif is strictly required for association of hU3-55K, which it also contacts directly through protein–protein interaction (PMID:12810916, PMID:12381732, PMID:16782898). Beyond RNA folding, distinct surface regions of 15.5K present separate protein–protein interaction sites that select among U4/U6, C/D snoRNP, and U3 B/C assembly pathways (PMID:16782898). 15.5K also engages the assembly factor Rsa1p/NUFIP through a defined electrostatic and hydrophobic interface; because this surface overlaps with the contacts used in mature, active snoRNPs, the interaction is mutually exclusive with snoRNP activation and is required for normal cell growth and snoRNP formation (PMID:24234454).

1. 2002 High

   Established that 15.5K is the nucleating RNA-binding factor whose binding is a prerequisite for ordered assembly of box C/D snoRNP proteins, defining a hierarchical assembly principle.

   Evidence In vitro assembly with purified components, microinjection of snoRNA into HeLa cells, and stem II mutagenesis

   PMID:12417735

   Open questions at the time

   • Did not resolve the structural basis of K-turn recognition
   • Mechanism by which stem II recruits NOP56/NOP58/fibrillarin not defined
2. 2002 High

   Extended the nucleation principle to the U3 snoRNP by showing that 15.5K binding to the box B/C motif is strictly required for hU3-55K recruitment, with direct RNA cross-linking and a shared K-turn-like conformation.

   Evidence In vitro assembly, UV cross-linking, and deletion/mutagenesis mapping of U3BC RNA

   PMID:12381732

   Open questions at the time

   • Did not identify the protein surface mediating hU3-55K recruitment
   • Role of the conserved flanking structure mechanistically undefined
3. 2003 High

   Showed 15.5K/Snu13p binds two distinct K-turns within U3 snoRNA with differing affinity and sequence constraints, both required for U3 snoRNP assembly and activity.

   Evidence Chemical probing of reconstituted complexes, gel-shift assays, phylogenetics, and yeast functional assays with base-substituted U3

   PMID:12810916

   Open questions at the time

   • Basis for higher B/C affinity despite lacking a normal stem I not structurally resolved
   • How dual binding is coordinated during assembly unknown
4. 2005 Medium

   Defined the recognition mechanism as induced fit: the cognate U4 K-turn RNA is flexible and unstructured until 15.5K folds and stabilizes the kinked conformation.

   Evidence Molecular dynamics simulation with chemical RNA modification validation

   PMID:15659359

   Open questions at the time

   • Computational with limited experimental validation
   • Did not test the model across all cognate K-turn substrates
5. 2006 High

   Demonstrated that 15.5K contributes to assembly beyond RNA binding, using distinct surface regions to present complex-specific protein–protein interaction sites, including a direct contact with hU3-55K.

   Evidence Systematic surface mutagenesis, in vitro RNP assembly, and direct protein–protein interaction assays

   PMID:16782898

   Open questions at the time

   • Did not map full interactomes at each surface
   • Structural detail of partner docking not resolved
6. 2007 Medium

   Provided the free-protein NMR structure showing 15.5K is well-ordered with flexibility localized to helix α3, implicating that local dynamics in K-turn recognition.

   Evidence Solution NMR structure, 15N relaxation, and H/D exchange

   PMID:18044964

   Open questions at the time

   • No mutagenesis linking α3 flexibility to binding
   • Functional consequence of dynamics not tested
7. 2013 High

   Resolved how premature snoRNP activation is avoided: defined the Snu13p–Rsa1p/NUFIP interface and showed it is mutually exclusive with active snoRNP contacts and required for growth and snoRNP formation.

   Evidence NMR structure, docking, biophysical binding, site-directed mutagenesis, and yeast functional assays

   PMID:24234454

   Open questions at the time

   • Timing of the Rsa1p-to-active-snoRNP handoff not directly observed
   • Other assembly factors at this surface not enumerated
8. 2016 Medium

   Confirmed structural conservation of the Snu13 fold across highly divergent eukaryotes using a minimal spliceosome ortholog.

   Evidence X-ray crystallography of Cyanidioschyzon merolae Snu13

   PMID:26833716

   Open questions at the time

   • No functional mutagenesis in this system
   • Did not test divergent partner interactions
9. 2019 Medium

   Refined the induced-fit model by showing substrate-specific RNA energetics: the C/D U14 K-turn is pre-folded in solution whereas the U4 K-turn requires protein or ions, with Snu13p stabilizing both.

   Evidence Solution NMR structure of kt-U14 ± Snu13p binding

   PMID:30914254

   Open questions at the time

   • Functional consequence of pre-folding for assembly kinetics untested
   • Generality across other C/D K-turns not established
10. 2025 Medium

    Showed the Snu13–U4 snRNA interaction is pharmacologically tractable, with small molecules competitively inhibiting K-turn RNA binding.

    Evidence Small molecule microarray screening, deltaSHAPE, IC50 inhibition assay, and MD simulation (preprint)

    PMID:bio_10.1101_2025.10.10.681657

    Open questions at the time

    • Preprint, single lab
    • Cellular and selectivity effects against other K-turn complexes not demonstrated

• How the timing of the switch from assembly-factor-bound (Rsa1p/NUFIP) intermediates to mature, catalytically engaged snoRNPs is regulated in cells remains unresolved.
• No in-cell kinetic trajectory of the assembly-to-activation handoff
• Regulatory signals controlling surface occupancy unknown