SLBP has been cloned from humans, C. elegans, D. melanogaster, X. laevis, and sea urchins. The full length human protein has 270 amino acids (31 kDa) with a centrally located RNA binding domain (RBD). The 75 amino acid RBD is well conserved across species, however the remainder of SLBP is highly divergent in most organisms and not homologous to any other protein in the eukaryotic genomes.
Function
This gene encodes a protein that binds to the histone 3' UTR stem-loop structure in replication-dependent histonemRNAs. Histone mRNAs do not contain introns or polyadenylation signals, and are processed by a single endonucleolytic cleavage event downstream of the stem-loop. The stem-loop structure is essential for efficient processing of the histone pre-mRNA but this structure also controls the transport, translation and stability of histone mRNAs. SLBP expression is regulated during S-phase of the cell cycle, increasing more than 10-fold during the latter part of G1.
All SLBP proteins are capable of forming a highly stable complex with histone stem-loop RNA. Complex formation with the histone mRNA stem-loop is achieved by a novel three-helix bundle fold. SLBP proteins also recognize the tetraloop structure of the histone hairpin, the base of the stem, and the 5' flanking region. The crystal structure of human SLBP in complex with the stem-loop RNA as well as the exonucleaseEri1 reveals that the Arg181 residue of SLBP specifically interacts with the second guanine base in the RNA stem.[4] The rest of the protein is intrinsically disordered in fruit-flies as well as in humans. A unique feature of the SLBP RBD is that it is phosphorylated in its RNA binding domain at the Thr171 residue. The SLBP RBD also undergoes proline isomerization about this sequence and is a substrate for the prolyl isomerase Pin1. The N-terminal domain of human SLBP is required for translation activation of histone mRNAs via its interaction with SLIP1. SLBP also interacts with the CBP80 associated protein CTIF to facilitate rapid degradation of histone mRNAs. SLBP is a phosphoprotein and besides T171, it is also phosphorylated at Ser7, Ser20, Ser23, Thr60, Thr61 in mammalian cells. The phosphorylation at Thr60 is mediated by CK2 and Thr61 is by Cyclin A/Cdk1.[3]
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Choe J, Mi Kim K, Park S, Kyung Lee Y, Song O-K, Kim MK, Lee BG, Song HK, Kim YK (2013). "Rapid degradation of replication-dependent histone mRNAs largely occurs on mRNAs bound by nuclear cap-binding proteins 80 and 20". Nucleic Acids Research. 41 (2): 1307–1318. doi:10.1093/nar/gks1196.
Thapar R, Marzluff WF, Redinbo MR (July 2004). "Electrostatic contribution of serine phosphorylation to the Drosophila SLBP--histone mRNA complex". Biochemistry. 43 (29): 9401–12. doi:10.1021/bi036315j. PMID15260483.
Thapar R, Mueller GA, Marzluff WF (July 2004). "The N-terminal domain of the Drosophila histone mRNA binding protein, SLBP, is intrinsically disordered with nascent helical structure". Biochemistry. 43 (29): 9390–400. doi:10.1021/bi036314r. PMID15260482.
Wang ZF, Whitfield ML, Ingledue TC, Dominski Z, Marzluff WF (December 1996). "The protein that binds the 3' end of histone mRNA: a novel RNA-binding protein required for histone pre-mRNA processing". Genes Dev. 10 (23): 3028–40. doi:10.1101/gad.10.23.3028. PMID8957003.
Allard P, Champigny MJ, Skoggard S, Erkmann JA, Whitfield ML, Marzluff WF, Clarke HJ (December 2002). "Stem-loop binding protein accumulates during oocyte maturation and is not cell-cycle-regulated in the early mouse embryo". J. Cell Sci. 115 (Pt 23): 4577–86. doi:10.1242/jcs.00132. PMID12415002.
Zhao X, McKillop-Smith S, Müller B (December 2004). "The human histone gene expression regulator HBP/SLBP is required for histone and DNA synthesis, cell cycle progression and cell proliferation in mitotic cells". J. Cell Sci. 117 (Pt 25): 6043–51. doi:10.1242/jcs.01523. PMID15546920.