Poly(C)-binding proteins as transcriptional regulators of gene expression

Abstract

Poly(C)-binding proteins (PCBPs) are generally known as RNA-binding proteins that interact in a sequence-specific fashion with single-stranded poly(C). They can be divided into two groups: hnRNP K and PCBP1-4. These proteins are involved mainly in various posttranscriptional regulations (e.g., mRNA stabilization or translational activation/silencing). In this review, we summarize and discuss how PCBPs act as transcriptional regulators by binding to specific elements in gene promoters that interact with the RNA polymerase II transcription machinery. Transcriptional regulation of PCBPs might itself be regulated by their localization within the cell. For example, activation by p21-activated kinase 1 induces increased nuclear retention of PCBP1, as well as increased promoter activity. PCBPs can function as a signal-dependent and coordinated regulator of transcription in eukaryotic cells. We address the molecular mechanisms by which PCBPs binding to single-and double-stranded DNA mediates gene expression. KeywordsPoly(C)-binding proteins; p21-activated kinase 1; DNA-binding proteins; Transcriptional regulationThe poly(C)-binding proteins (PCBPs) are characterized by high affinity for, and sequencespecific interaction with polycytosine, poly(C). In mammalian cells, these PCBPs belong to one of two subsets: hnRNP K/J, or the alpha-complex proteins (e.g., PCBP1-4) [1]. hnRNP K, PCBP1, and PCBP2 have been studied in the greatest detail. The latter two proteins are also known as αCP1 and αCP2, or hnRNPE1 and hnRNPE2 [2,3]. Recently, two other members of the αCP family were discovered: PCBP3 (αCP3) and PCBP4 (αCP4) [4].PCBPs are expressed broadly in human and mouse tissues and demonstrate poly(C)-binding specificity [2,4,5]. All members of the PCBP family are related evolutionarily. The common feature of all PCBPs is the presence of three hnRNP K homology (KH) domains [1]; these are RNA-binding modules of about 70 amino acids in length. PCBP1 and PCBP2 share the highest level of amino acid sequence similarity (89%) [6]. PCBP3 is more divergent, and PCBP4 is the most distantly related (52% divergence from PCBP2 [4,7]. Members of this family perform multiple functions through their poly(C)-binding ability, including mRNA stabilization [8- Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Structure of PCBPsThe PCBPs contain three KH domains, two consecutive KH domains at the amino terminus and a third KH domain at the carboxyl terminus, separated by an intervening sequence of variable length (Fig. 1A). The structure of each KH domain consists of three α-helices...

The antiapoptotic protein, FLIP, is regulated by heterogeneous nuclear ribonucleoprotein K and correlates with poor overall survival of nasopharyngeal carcinoma patients

“…However, we did not find evidence that hnRNP K affects the stability of FLIP mRNA (Figure 3c), although FLIP mRNA could be regulated by IRF8 through mRNA stabilization. 37 Instead, similar to the hnRNP K-mediated induction of c-myc, Src and eIF4E gene expression through binding of poly(C) elements in their gene promoters, 1 we herein show that the poly(C) element of the FLIP promoter is important for its recognition and activation by both hnRNP K and nucleolin.…”

“…Analysis of the FLIP promoter sequence revealed four potential hnRNP K-binding poly(C) or C-C-C-C elements, R1 to R4 (Figure 3a). 1 To distinguish between transcriptional activation and post-transcriptional regulation, we examined the effect of hnRNP K knockdown on FLIP promoter activity and mRNA stability. We transfected a FLIP promoter reporter construct (À1179 to þ 281) into hnRNP K-knockdown NPC-TW02 cells and examined reporter activity 24 h later.…”

“…3 HnRNP K is a nucleocytoplasmic shuttling protein that is primarily located in the nucleus for transcriptional regulation. 1 However, cytoplasmic accumulation of hnRNP K through ERK-mediated phosphorylation of hnRNP K serines-284 and -353 has been reported in cervical carcinoma HeLa, 7 chronic myelogenous leukemia 8 and NPC 9 cells. The tumorigenic activity of hnRNP K appears to be conferred through its ability to increase proliferation, 10 antiapoptotic effects, 9 clonogenic potential 8 and metastasis.…”

Dissecting noncoding and pathogen RNA–protein interactomes

“…We applied this Fully Automated and Standardized iCLIP (FAST-iCLIP) pipeline to a published iCLIP data set of the RBP hnRNP-C, recapitulating the published biology of this protein. We then applied FAST-iCLIP to the human and viral interactomes of Poly-C binding protein 2 (PCBP2), a KH domain (hnRNP-K homology) (Lunde et al 2007) containing RBP that has a preference for C/Urich motifs (Choi et al 2009) as well as pathological associations in cancer (Eiring et al 2010;Han et al 2013) and infectious disease (Gamarnik and Andino 2000;Herold and Andino 2001;Shetty et al 2013). The unbiased characterization of the PCBP2 protein revealed novel RNAprotein interactions and provided supporting evidence for previously known cellular and viral roles for PCBP2.…”

“…PCBP2 has a preference for C-rich motifs (Choi et al 2009) and notable association with infectious disease (Han et al 2013). PCBP2 was a favorable target, as features of its biology are well understood, but no CLIP-seq study has been reported to date (Waggoner and Liebhaber 2003;Han et al 2013).…”