Structurally conserved and functionally divergent yeast Ssu72 phosphatases

Ssu72: a versatile protein with functions in transcription and beyond

Eukaryotic transcription is a complex process involving a vast network of protein and RNA factors that influence gene expression. The main player in transcription is the RNA polymerase that synthesizes the RNA from the DNA template. RNA polymerase II (RNAPII) transcribes all protein coding genes and some noncoding RNAs in eukaryotic cells. The polymerase is aided by interacting partners that shuttle it along the gene for initiation, elongation and termination of transcription. One of the many factors that assist RNAPII in transcription of genes is Ssu72. It is a carboxy-terminal-domain (CTD)-phosphatase that plays pleiotropic roles in the transcription cycle. It is essential for cell viability in Saccharomyces cerevisiae, the organism in which it was discovered. The homologues of Ssu72 have been identified in humans, mice, plants, flies, and fungi thereby suggesting the evolutionarily conserved nature of the protein. Recent studies have implicated the factor beyond the confines of transcription in homeostasis and diseases.

circSSU72 Promotes Cell Proliferation, Migration and Invasion of Papillary Thyroid Carcinoma Cells by Targeting miR-451a/S1PR2 Axis

Introduction: Thyroid cancer is the most common endocrine malignancy with Papillary Thyroid Carcinoma (PTC) as the most common pathological type. Due to low mortality but a high incidence, PTC still causes a relatively heavy burden on financial costs, human health, and quality of life. Emerging researches have indicated that circular RNAs (circRNAs) play a significant regulatory role in various cancers, including PTC. However, the functions and mechanisms of circRNAs derived from SSU72 remain unknown.

Method: The expression level of circRNAs derived from the exons of SSU72, miR-361–3p, miR-451a, and S1PR2 was evaluated by qRT-PCR assay or western blot assay. The interactions between circSSU72 (hsa_circ_0009294), miR-451a, and S1PR2 were verified by dual-luciferase reporter assay. Effects of circSSU72, miR-451a, and S1PR2 on cell proliferation, migration, and invasion were confirmed by colony formation assay, cell counting kit-8 (CCK-8), wound healing assay, and Transwell assays in vitro.

Results: circSSU72 was upregulated in PTC; circSSU72 knockdown inhibited PTC cell proliferation, migration, and invasion. In addition, circSSU72 could negatively regulate miR-451a by functioning as a sponge. circSSU72 promoted PTC cell proliferation, migration, and invasion by targeting miR-451a in vitro. We further found that miR-451a inhibited PTC cell proliferation, migration, and invasion by regulating S1PR2. Overall, the circSSU72/miR-451a/S1PR2 axis might influence PTC cell proliferation, migration, and invasion.

Conclusions: Overall, circSSU72 (hsa_circ_0009294)/miR-451a/S1PR2 axis may promote cell proliferation, migration, and invasion in PTC. Thus, circSSU72 may serve as a potential biomarker and therapeutic target for PTC.

Ssu72 Dual-Specific Protein Phosphatase: From Gene to Diseases

More than 70% of eukaryotic proteins are regulated by phosphorylation. However, the mechanism of dephosphorylation that counteracts phosphorylation is less studied. Phosphatases are classified into 104 distinct groups based on substrate-specific features and the sequence homologies in their catalytic domains. Among them, dual-specificity phosphatases (DUSPs) that dephosphorylate both phosphoserine/threonine and phosphotyrosine are important for cellular homeostasis. Ssu72 is a newly studied phosphatase with dual specificity that can dephosphorylate both phosphoserine/threonine and phosphotyrosine. It is important for cell-growth signaling, metabolism, and immune activation. Ssu72 was initially identified as a phosphatase for the Ser5 and Ser7 residues of the C-terminal domain of RNA polymerase II. It prefers the cis configuration of the serine-proline motif within its substrate and regulates Pin1, different from other phosphatases. It has recently been reported that Ssu72 can regulate sister chromatid cohesion and the separation of duplicated chromosomes during the cell cycle. Furthermore, Ssu72 appears to be involved in the regulation of T cell receptor signaling, telomere regulation, and even hepatocyte homeostasis in response to a variety of stress and damage signals. In this review, we aim to summarize various functions of the Ssu72 phosphatase, their implications in diseases, and potential therapeutic indications.

Diverse and conserved roles of the protein Ssu72 in eukaryotes: from yeast to higher organisms

Gene transcription is a complex biological process that involves a set of factors, enzymes and nucleotides. Ssu72 plays a crucial role in every step of gene transcription. RNA polymerase II (RNAPII) occupies an important position in the synthesis of mRNAs. The largest subunit of RNAPII, Rpb1, harbors its C-terminal domain (CTD), which participates in the initiation, elongation and termination of transcription. The CTD consists of heptad repeats of the consensus motif Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 and is highly conserved among different species. The CTD is flexible in structure and undergoes conformational changes in response to serine phosphorylation and proline isomerization, which are regulated by specific kinases/phosphatases and isomerases, respectively. Ssu72 is a CTD phosphatase with catalytic activity against phosphorylated Ser5 and Ser7. The isomerization of Pro6 affects the binding of Ssu72 to its substrate. Ssu72 can also indirectly change the phosphorylation status of Ser2. In addition, Ssu72 is a member of the 3'-end cleavage and polyadenylation factor (CPF) complex. Together with other CPF components, Ssu72 regulates the 3'-end processing of premature mRNA. Recent studies have revealed other roles of Ssu72, including its roles in balancing phosphate homeostasis and controlling chromosome behaviors, which should be further explored. In conclusion, the protein Ssu72 is an enzyme worthy of attention, not confined to its role in gene transcription.

Crosstalk of promoter and terminator during RNA polymerase II transcription cycle

The transcription cycle of RNAPII is comprised of three consecutive steps; initiation, elongation and termination. It has been assumed that the initiation and termination steps occur in spatial isolation, essentially as independent events. A growing body of evidence, however, has challenged this dogma. First, factors involved in initiation and termination exhibit both a genetic and a physical interaction during transcription. Second, the initiation and termination factors have been found to occupy both ends of a transcribing gene. Third, physical interaction of initiation and termination factors occupying distal ends of a gene sometime results in the entire terminator region of a genes looping back and contact its cognate promoter, thereby forming a looped gene architecture during transcription. A logical interpretation of these findings is that the initiation and termination steps of transcription do not occur in isolation. There is extensive communication of factors occupying promoter and terminator ends of a gene during transcription cycle. This review entails a discussion of the promoter-terminator crosstalk and its implication in the context of transcription.