Conserved and specific functions of mammalian ssu72.
We describe the cloning and characterization of a human homolog of the yeast transcription/RNA-processing factor Ssu72, following a yeast two-hybrid screen for pRb-binding factors in the prostate gland. Interaction between hSsu72 and pRb was observed in transfected mammalian cells and involved multiple domains in pRb; however, so far, mutual effects of these two factors could not be demonstrated. Like the yeast counterpart, mammalian Ssu72 associates with TFIIB and the yeast cleavage/polyadenylation factor Pta1, and exhibits intrinsic phosphatase activity. Mammals contain a single ssu72 gene and a few pseudogenes. During mouse embryogenesis, ssu72 was highly expressed in the nervous system and intestine; high expression in the nervous system persisted in adult mice and was also readily observed in multiple human tumor cell lines. Both endogenous and ectopically expressed mammalian Ssu72 proteins resided primarily in the cytoplasm and only partly in the nucleus. Interestingly, fusion to a strong nuclear localization signal conferred nuclear localization only in a fraction of transfected cells, suggesting active tethering in the cytoplasm. Suppression of ssu72 expression in mammalian cells by siRNA did not reduce proliferation/survival, and its over-expression did not affect transcription of candidate genes in transient reporter assays. Despite high conservation, hssu72 was unable to rescue an ssu72 lethal mutation in yeast. Together, our results highlight conserved and mammalian specific characteristics of mammalian ssu72.
St-Pierre
Benoit
B
Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network 67 College Street, Room 407, Toronto, Ontario, Canada M5G 2m1.
Liu
Xudong
X
Kha
Lan-Chau T
LC
Zhu
Xudong
X
Ryan
Owen
O
Jiang
Zhe
Z
Zacksenhaus
Eldad
E
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002352:Carrier Proteins / Q000032:analysis / Q000235:genetics* / Q000502:physiology*; D048708:Cell Growth Processes; D002467:Cell Nucleus / Q000737:chemistry; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D003593:Cytoplasm / Q000737:chemistry; D018507:Gene Expression Regulation, Developmental; D005816:Genetic Complementation Test; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D016160:Retinoblastoma Protein / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D035581:Transcription Factor TFIIB / Q000378:metabolism; D015533:Transcriptional Activation; D020798:Two-Hybrid System Techniques; D039221:mRNA Cleavage and Polyadenylation Factors / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*
D002352:Carrier Proteins; D019913:Nuclear Localization Signals; C076229:PTA1 protein, S cerevisiae; D012333:RNA, Messenger; D016160:Retinoblastoma Protein; C416934:SSU72 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D035581:Transcription Factor TFIIB; D039221:mRNA Cleavage and Polyadenylation Factors; D010749:Phosphoprotein Phosphatases; C496300:SSU72 protein, human