LSM6

LSM6 encodes a conserved Sm-like protein that adopts the canonical Sm fold and functions as a building subunit of heptameric LSm rings governing U6 snRNA biogenesis, pre-mRNA splicing, and cytoplasmic mRNA decay (PMID:22001694, PMID:22615807). It assembles with LSM5 and LSM7 into a defined hexameric sub-complex (LSm657, organized Lsm5–Lsm6–Lsm7) that serves as an assembly intermediate en route to the native LSm1-7 and LSm2-8 heptamers and that binds oligo(U) RNA (PMID:22001694, PMID:22615807). Within the nuclear LSm2-8 ring, LSM6 contributes to stabilization of nascent U6 snRNA, acting redundantly with the La protein Lhp1p (PMID:11333229), while genetic interactions place it additionally in the cytoplasmic P-body/mRNA decay pathway (PMID:20230609). As part of the SC2 (Lsm6/5/7) module of the ring, LSM6 participates in inter-subcomplex interfaces whose strength dictates whether a ring behaves as an Sm-type scaffold or an Lsm-type chaperone (PMID:40433979). LSM6 also directly interacts with the spinal muscular atrophy protein SMN through arginine- and glycine-rich sequences, linking it to snRNP assembly (PMID:10851237).

1. 2000 Medium

   Established a physical link between LSM6 and the snRNP-assembly machinery by showing it is among the U6-associated Sm-like proteins bound directly by the SMN protein.

   Evidence Co-immunoprecipitation / direct binding assay mapping the SMN interaction to arginine/glycine-rich domains

   PMID:10851237

   Open questions at the time

   • Single Co-IP/pulldown without reciprocal validation
   • Functional consequence of the SMN interaction for snRNP assembly not tested
   • Stoichiometry and ring context of the interaction unresolved
2. 2001 Medium

   Placed LSM6 functionally within the Lsm2-8 ring by showing its deletion makes the La protein Lhp1p essential, defining a redundant pathway for stabilizing nascent U6 snRNA.

   Evidence Genetic epistasis / deletion analysis in S. cerevisiae

   PMID:11333229

   Open questions at the time

   • Does not isolate LSM6's individual contribution from the rest of the ring
   • Mechanism of U6 stabilization at the molecular level not resolved
   • Yeast genetics; human relevance inferred
3. 2010 Low

   Extended LSM6's functional reach to the cytoplasmic mRNA decay pathway through genetic interaction with SUS1, LSM1, and PAT1.

   Evidence Genetic interaction / synthetic lethality assays in S. cerevisiae

   PMID:20230609

   Open questions at the time

   • Genetic interaction only, no direct biochemical role demonstrated
   • Cannot distinguish ring-level from subunit-level effects
   • No defined molecular activity assigned in this context
4. 2008 Medium

   Identified the protein-protein contacts used to recruit LSM6 into mixed ring scaffolds, showing C-terminal tail beta-sheet interactions drive subunit assembly.

   Evidence Crystal structure of Lsm3 plus pulldown of Lsm6/2/5 from yeast lysate

   PMID:18329667

   Open questions at the time

   • Recruitment shown via homomeric Lsm3 scaffold, not the native heteromeric ring
   • Order of subunit addition not fully defined
   • RNA-dependence of recruitment not addressed
5. 2011 High

   Defined the LSm657 hexamer as a discrete assembly intermediate and confirmed LSM6 adopts the canonical Sm fold positioned analogously to its Sm counterparts.

   Evidence X-ray crystallography at 2.5 Å, NMR, and pulldown assembly assays

   PMID:22001694

   Open questions at the time

   • RNA binding by the intermediate not characterized in this study
   • Path from LSm657 to full heptamers structurally inferred
6. 2012 High

   Established the subunit order Lsm5–Lsm6–Lsm7 and demonstrated the sub-complex binds oligo(U) RNA, assigning an RNA-binding function to the LSM6-containing module.

   Evidence X-ray crystallography, analytical ultracentrifugation, and EMSA on the S. pombe Lsm5/6/7 sub-complex

   PMID:22615807

   Open questions at the time

   • Contribution of LSM6 itself versus partners to RNA contact not dissected
   • Affinity/specificity for physiological U6 not quantified
7. 2025 High

   Revealed the mechanistic determinant distinguishing Sm-type scaffold from Lsm-type chaperone rings, with LSM6 as part of the SC2 module whose interface strength controls ring identity.

   Evidence Mutagenesis of subunit interfaces, biochemical reconstitution, and functional ring-conversion assays

   PMID:40433979

   Open questions at the time

   • In vivo relevance of engineered ring interconversion not established
   • LSM6's specific energetic contribution to the SC2 interface not isolated

• How LSM6's SMN interaction and its dual nuclear/cytoplasmic ring functions are coordinated during snRNP assembly and mRNA decay in human cells remains unresolved.
• No human cellular assay connecting SMN binding to ring assembly
• Regulation of LSm1-7 versus LSm2-8 partitioning unknown
• LSM6-specific (vs ring-level) function not separated