English
The precise and well-ordered interactions between nuclear precursor messenger RNAs (pre-mRNA) and the small nuclear RNAs (snRNA), U1, U2, U4, U5, and U6, are essential for intron removal from pre-mRNAs as well as for regulation of gene expression in eukaryotes. Although the assembly of the spliceosome is well known, the understanding of the maturation steps of their subcomponents lack behind. In Saccharomyces cerevisiae, all unprocessed snRNAs, so called premature snRNAs (pre-snRNA), must undergo nuclear export and the re-import (Becker et al., 2019). Inhibition of nuclear export causes pre-snRNAs to be incorporated into the spliceosome, resulting in splicing defects (Becker et al., 2019). Thus, the proper shuttling process of all snRNAs in yeast is a prerequisite to avoid splicing defects. Loading of the Sm-ring onto U1, U2, U4 and U5 occurs in the cytoplasm and is important for subsequent processing steps (Becker et al., 2019).
U6 differs from the other snRNAs in that it is the only one transcribed by RNA polymerase III (RNAP III), not RNA polymerase II (RNAP II), resulting in different features. It does not contain a 7-methylguanosine cap (m7G cap), a poly(A) tail and an Sm-ring binding site, which are crucial determinants for the shuttling process of other snRNAs. Therefore, it is currently unclear, how and why U6 shuttles into the cytoplasm.
In contrast to U1 to U5, U6 has an Lsm-ring binding site. The formation of U4 and U6 duplex (di-snRNA) is an essential step during U6 biogenesis. Previously, we have shown that U4 and U6 mislocalize in the cytoplasm of the lsm8-1 mutant in which the Lsm-ring is not properly assembled, suggesting that the U4/U6 di-snRNA formation might occur in the cytoplasm. Therefore, the U4/U6 di-snRNA formation in the cytoplasm may contribute to the shuttling process of U6. In the nucleus, we reveal that Lhp1, an associated protein of unprocessed U6 (pre-U6) and unprocessed U4 (pre-U4) that protects the newly synthesized snRNAs from degradation, serves as an export adaptor for nuclear export by interacting with the export receptor Mex67. It also interacts with the three guard proteins Npl3, Gbp2 and Hrb1. Indeed, we show that the formation of pre-U4 and pre-U6 small nuclear ribonucleoprotein (di-snRNP) via Prp24, a component of pre-U6 small nuclear ribonucleoproteins (snRNP), occurs in the cytoplasm. Importantly, a surveillance mechanism, which is mediated by the three mRNA guard proteins Npl3, Gbp2 and Hrb1, is responsible for cytoplasmic quality control of the correctly formed pre-U4/pre-U6 duplex. Unpaired pre-U4 and pre-U6 undergo a cytoplasmic degradation mediated by those of guard proteins. The guard proteins Gbp2 and Hrb1 recruit the cytoplasmic decapping factors Dcp1 and Dcp2 for 5’ -decapping. Subsequently, the three guard proteins Npl3, Gbp2 and Hrb1 recruit the cytoplasmic 5' to 3' exonuclease Xrn1 for 5’ -mediated exonucleolytic degradation. Only intact pre-U4 and pre-U6 di-snRNPs are re-imported into the nucleus by the Lsm-ring, a component of pre-U6 snRNPs, which interacts with the import receptors Cse1 and Mtr10. Thus, contact of defective di-snRNPs with the spliceosome is prevented by the guard protein-mediated degradation mechanism in the cytoplasm.
Keywords: snRNA maturation; snRNA processing; Prp24; guard protein
Persistent Address:
http://dx.doi.org/10.53846/goediss-11407
