Chicken Y-box proteins chk-YB-1b and chk-YB-2 repress translation by sequence-specific interaction with single-stranded RNA

Y-Box proteins comprise a large family of multifunctional proteins with a wide spectrum of activities in both transcription and translational regulation of gene expression. Earlier, we have reported on the involvement of chk-YB-2 in transcriptional regulation of Rous sarcoma virus long terminal repeats and the involvement of chk-YB-1b in transcriptional regulation of alpha1(I) collagen genes. Here, we have investigated the potential role of chk-YB-2 and chk-YB-1b in RNA metabolism. We report that chk-YB-2 and chk-YB-1b are localized predominantly in the cytoplasm and that they both can bind single-stranded RNA in a sequence-specific and reversible manner. Well-conserved cold-shock domain, N-terminal proline-rich domain and the alternating clusters of acidic and basic amino acids located in the C-terminal ends of these two proteins were all found to be necessary for their RNA-binding ability. Further, we demonstrate that these two proteins inhibit translation in vitro and that binding to RNA is required for this inhibition. The significance of these results is discussed.

Angiotensin II and extracellular matrix homeostasis

As a circulating hormone, endocrine properties of angiotensin (Ang) II are integral to circulatory homeostasis. Produced de novo its autocrine/paracrine properties contribute to biologic responses involving various connective tissues (e.g. extracellular matrix, adipose tissue, bone and its marrow). In this brief review, we develop the concept of extracellular matrix homeostasis, a self regulation of cellular composition and structure, wherein fibroblast-derived AngII regulates elaboration of TGF-beta 1, a fibrogenic cytokine responsible for connective tissue formation at normal and pathologic sites of collagen turnover.

chk-YB-1b, a Y-box binding protein activates transcription from rat alpha1(I) procollagen gene promoter

Type-I collagen, the predominant component of extracellular matrix, is a triple-helical protein consisting of two alpha1 polypeptides and one alpha2 polypeptide. Expression of alpha1 and alpha2 procollagen genes is co-ordinately regulated under both normal and various pathological conditions. However, the basis of this co-ordinate regulation is not well known. YB-1b, a Y-box protein, has been shown to bind to the polypyrimidine tract present in the alpha2 procollagen gene. Here, we show that chk-YB-1b, a YB-1 homologue, binds in a single-strand-sequence-specific manner to the highly conserved pyrimidine-rich sequences in both alpha1(I) and alpha2(I) procollagen promoters from different species, as demonstrated by electrophoretic-mobility-shift assays and by DNaseI footprinting experiments. Transiently transfected and retrovirally expressed antisense oligonucleotides directed against chk-YB-1b specifically inhibited the alpha1(I) procollagen promoter-driven transcription in cultured fibroblasts. Considering these data and the fact that the chk-YB-1b binding site is one of the few sites between alpha1(I) and alpha2(I) procollagen promoters that is conserved from chicken to human, it is proposed that chk-YB-1b may be involved in co-ordinate expression of these two collagen genes.

Retroviral vectors for gene therapy

Although there are many hurdles to overcome for successful gene therapy, there is a vast potential to permanently incorporate genes into cells to correct genetic disorders and to combat viral infections. Retroviruses, inspite of some limitations, offer the best hope in this direction and lentiviral vectors, which infect nondividing cells, may be the choice in the future, especially in gene therapy for central nervous system disorders.

Role of single-stranded DNA regions and Y-box proteins in transcriptional regulation of viral and cellular genes

Single-stranded regions, known to be important for optimal rates of transcription, have been observed in the promoters of several cellular genes as well as in the promoters of many pathogenic viruses. Several host-encoded, single-stranded DNA binding proteins capable of binding these regions have been purified and their genes isolated. In this review, information available about single-stranded regions present within various promoters and the interaction of a novel class of single-stranded DNA binding transcription factors belonging to the Y-box family of proteins is reviewed. Mechanisms by which these proteins influence transcription of both cellular and viral genes are proposed.

Characterization of the DNA-binding domain of the avian Y-box protein, chkYB-2, and mutational analysis of its single-strand binding motif in the Rous sarcoma virus enhancer

chkYB-2 is a sequence-specific, single-stranded DNA binding chicken Y-box protein that promotes Rous sarcoma virus long terminal repeat (RSV LTR)-driven transcription in avian fibroblasts. The DNA-binding domain of chkYB-2 has been mapped by characterizing the DNA binding properties of purified recombinant chkYB-2 mutant polypeptides. The data indicate that the invariant cold shock domain (CSD) is necessary but not sufficient for association with DNA and suggest that another conserved region, adjacent to the carboxyl boundary of the CSD, plays a role in high-affinity DNA binding. chkYB-2 binds to a tandem repeat of the 5'-GTACCACC-3' motif on the RSV LTR. Mutational analysis of this recognition sequence revealed the requirement of an essentially unaltered template for both high-affinity binding by chkYB-2 as well as maximal transcriptional activity of the RSV LTR in vivo. The single-stranded DNA binding activity of chkYB-2 is augmented by Mg2+. The possible significance of this finding for transactivation by a single-strand DNA binding protein is discussed.

Chicken YB-2, a Y-box protein, is a potent activator of Rous sarcoma virus long terminal repeat-driven transcription in avian fibroblasts

We have previously reported on the cloning and characterization of chk-YB-2, a novel member of the Y-box family of proteins, that binds to the sequence 5'-GTACCACC-3' present on the noncoding strand of the Rous sarcoma virus (RSV) long terminal repeat (LTR) in a single-strand-specific manner. Here, we demonstrate that deletion or mutation of this motif not only eliminates chk-YB-2 binding in vitro but also down-regulates RSV LTR-driven transcription in avian cells. Selective abrogation of chk-YB-2 expression by using antisense oligonucleotides decreased RSV LTR-driven transcription in a promoter-specific manner. This inhibition was not observed when a reporter construct with a deletion in the chk-YB-2 binding site was used. Depletion of cellular chk-YB-2 by transfecting the cells with excess of its recognition sequence oligonucleotides also resulted in reduced transcription from the RSV LTR. Taken together, these results suggest that chk-YB-2 acts as an activator of LTR-promoted transcription in avian cells and that this activation is mediated primarily through the sequence 5'-GTACCACC-3'.

Identification and characterization of promoter elements responsible for the induction of the albumin gene by heat shock in early embryonic rat liver

We had reported earlier that the expression of albumin increases upon heat shock in embryonic rat liver cells at about 12-13 days of gestation. Here, we report on the identification of heat shock elements (HSEs) within -450 bp of the rat albumin promoter using chloramphenicol acetyl transferase (CAT) assays done with the extracts from H4II-E-C3 cells transfected with plasmids carrying the CAT reporter gene under the control of different deletion fragments of the rat albumin promoter. Gel retardation assays done with synthetic oligonucleotides representing putative HSEs in the rat albumin promoter and H4II-E-C3 cell extracts show that the heat shock factors bind this region in a sequence-specific and reversible manner. Super-shift assays demonstrated that the HSEs present in the rat albumin promoter are bound by HSF1 and not by HSF2. This effect of heat shock on the expression of rat serum albumin is seen only in the liver and is not observed in other tissues, suggesting that HSF-mediated activation of albumin gene cannot overcome the negative regulatory factors present in other tissues. In addition to the HSEs, we have identified a putative GAGA factor binding site in the rat albumin promoter at -228 bp to -252 bp position. These GAGA repeats are bound in a sequence-specific and reversible manner by two factors in a nonstressed cell, whereas only one of these two factors continues to bind the GAGA repeats under heat shock conditions. The physiological significance of these results is discussed.

Cloning of Rous sarcoma virus enhancer factor genes. II. RSV-EF-II, abundantly expressed in fibroblasts and muscle tissue, binds to an octamer sequence, 5'-GTACCACC-3', in the noncoding strand of RSV enhancer

Rous sarcoma virus (RSV) mainly replicates in avian fibroblasts, and the U3 enhancer region of the long terminal repeats of RSV contains the determinants for its tissue-tropic expression. We describe the cloning and characterization of an avian gene that encodes a protein capable of binding to the enhancer region of Rous sarcoma virus. A PCR-derived probe corresponding to the U3 region of RSV was used to isolate a cDNA clone by screening a chicken cDNA expression library. The cDNA is predicted to encode a polypeptide of 298 amino acids that is homologous to the Y-box (inverted CCAAT) family of DNA-binding transcription factors. This factor, which we refer to as Rous sarcoma virus enhancer factor-II (RSV-EF-II), shows 99% aa identity over a 105-amino-acid stretch that is highly conserved in all Y-box proteins, and is commonly referred to as the cold shock domain. RSV-EF-II selectively binds to single-stranded DNA, and the binding site, as determined by electrophoretic mobility shift assays, consists of the sequence 5' GTACCACC 3' located between nucleotides -112 to -119 in the noncoding strand of the RSV enhancer. Although RSV-EF-II shares considerable homology with the Y-box family of proteins, it does not bind to the inverted CCAAT boxes at positions -65 to -69 and -129 to -133 in the RSV LTR. Northern analysis indicates that RSV-EF-II-specific transcripts are expressed predominantly in avian fibroblasts and muscle tissue. The results of these binding and mRNA expression expriments suggest that RSV-EF-II may play an important role in tissue- and host-specific expression of RSV LTR-driven gene expression. Further, we show that RSV-EF-II acts as a repressor of transcription.