Summary of Research Activity:
Repressor Loop Formation in Translation
In post-transcriptional regulation of mRNAs, RNA binding proteins (RBP) are important trans-acting regulators. RBPs work either as activator or repressor of translation. We are particulary interested in repressor loop formation by RBPs during translation repression. RBP vigilin is a repressor for c-fms mRNA translation. We propose that vigilin is involved in reperssor loop formation.
Our research indicates that both Vigilin and HuR work together to coordinate the translation of c-fms mRNA. Dysregulation of coordination between Vigilin and HuR can cause abnormal expression of c-fms mRNA.
Deadenylation and Bulk Translation
Rate of translation is one of the critical factors, determining protein abundance in a cell. After a pioneer round of translation in which 80S ribosome scans the linear mRNA, a cap-dependent mRNP closed-loop is formed for bulk translation. However, detailed analysis about the transition from linear to closed-loop formation is yet to be studied.
Deadenylated mRNAs are generally thought as a translationally inactive form. However, our data shows that nucleolin-induced, deadenylated mRNAs are associated with heavy polyribosomes, indicating deadenylated mRNAs are in the translationally active state. In general, newly synthesized mRNAs are either assembled in a translation initiation complex or deadenylation-decay complex. In mRNA decay, deadenylation is the major step followed by decapping and exonuclease digestion. However, mRNA deadenylation may have a previously unidentified function to enhance bulk translation. After a pioneer round of translation, a mRNP closed-loop is formed by joining the 5’-cap and 3’-poly(A)n tail by RNA-binding proteins. Binding of PABPC to the poly(A)n tail and to the eIF4G in eIF4F complex results in a mRNP closed-loop for bulk translation. PABPC is also involved in the microRNA-directed mRNA decay by scaffolding deadenylation complex. Importantly, we documented that nucleolin interacts with the PABPC and works together for deadenylation and mRNP closed-loop formation; i.e., we observed that nucleolin enhances the microRNA-directed deadenylation of CSF-1 mRNA, but increases the translation of CSF-1 mRNA. Nucleolin interacts to the Pan2/Pan3 and CCR4-NOT deadenylation complex, indicating nucleolin’s involvement in the mRNA deadenylation. Nucleolin also interacts with the eIF4F translation initiation complex, suggesting another role of nucleolin in translation initiation. We found that an increased level of deadenylated mRNAs is associated with heavy polyribosomes in nucleolin overexpressed cell lines. Together these results indicate that nucleolin-mediated deadenylation is not followed by mRNA decay, but can lead to the bulk translation by forming poly(A)n-less mRNP closed-loop.
We propose that a non-canonical mRNP closed-loop lacking poly(A)n tail can be formed by nucleolin for bulk translation. In this model, nucleolin enhances deadenylation of the mRNA and induces the formation of poly(A)n-less mRNP closed-loop by binding dsRNA formed between the mRNA 5’- and 3’-UTR complementary sequences. These complementary sequences are found in nucleolin-targeted mRNAs including CSF-1 and AKT1 mRNAs. Nucleolin is proposed as a mediator for the transition from the canonical to non-canonical poly(A)n-less mRNP closed-loop to enhance the bulk translation.