Initial spliceosomal U4/U6 di-snRNA formation occurs in the cytoplasm of Saccharomyces cerevisiae and requires a guard protein mediated quality control.

Unlike mRNA surveillance, ncRNA quality control is less well understood. While mRNA maturation is monitored by guard proteins that allow nuclear export of correctly processed transcripts or retention and degradation of faulty RNAs, such surveillance system is unknown for ncRNAs. This study investigates the maturation process of the snRNA U6 in Saccharomyces cerevisiae, revealing that this RNAPIII transcript undergoes quality control by established guard proteins and the novel factor Lhp1 (human La), which ensures proper loading of the Lsm-ring in the nucleus. Subsequent Mex67 binding facilitates the nuclear export of pre-U6. In the cytoplasm, pre-U6 associates with Prp24 which assists in annealing with pre-U4. Defects in di-snRNP formation are identified by the guard proteins Npl3, Gbp2, and Hrb1. These proteins retain the RNA in the cytoplasm and recruit Dcp1 and Dcp2 for de-capping, along with Xrn1 for degradation of faulty pre-U6. Correctly assembled U4/U6 complexes are released from the guard proteins and imported back into the nucleus. This guard protein-mediated surveillance mechanism prevents faulty di-snRNPs to torpedo the spliceosome, underscoring the significance of the compartmented maturation and quality control of ncRNA. Additionally, the study illustrates that RNA surveillance mechanisms extend beyond coding RNAs and involve similar quality control mechanisms and proteins.