UBP1, a novel hnRNP-like protein that functions at multiple steps of higher plant nuclear pre-mRNA maturation

Abstract

Efficient splicing of higher plant pre‐mRNAs depends on AU‐ or U‐rich sequences in introns. Moreover, AU‐rich sequences present in 3′‐untranslated regions (3′‐UTRs) may play a role in 3′ end processing of plant mRNAs. Here, we describe the cloning and characterization of a Nicotiana plumbaginifolia nuclear protein that can be cross‐linked to U‐rich intron and 3′‐UTR sequences in vitro, and associates with nuclear poly(A)+ RNA in vivo. The protein, UBP1, strongly enhances the splicing of otherwise inefficiently processed introns when overexpressed in protoplasts. It also increases the accumulation of reporter mRNAs that contain suboptimal introns or are intronless. The enhanced accumulation is apparently due to UBP1 interacting with the 3′‐UTR and protecting mRNA from exonucleolytic degradation. The effect on mRNA accumulation but not on mRNA splicing was found to be promoter specific. The fact that these effects of UBP1 can be separated suggests that they represent two independent activities. The properties of UBP1 indicate that it is an hnRNP protein that functions at multiple steps to facilitate the nuclear maturation of plant pre‐mRNAs.

Determinants of Plant U12-Dependent Intron Splicing Efficiency

“…However, overexpression of UBP1 with the wheat (Triticum aestivum) amylase U2 intron, which has a relatively low UA content and splices poorly (5 to 15%) in tobacco protoplasts , increased splicing efficiency to 85% (Figure 6). Thus, whereas UBP1 can enhance splicing efficiency of poorly spliced U2 introns, confirming the findings of Lambermon et al (2000), neither UBP1 nor RBP45 influenced U12 intron splicing directly.…”

“…RBP45 and UBP1 are hnRNP-like RNA binding proteins with affinity for U-rich sequences (Gniadkowski et al, 1996;Lambermon et al, 2000;Lorković et al, 2000). UBP1 is able to improve the splicing efficiency of otherwise poorly spliced U2 plant introns .…”

“…The role of UA-rich elements in plant splicing is still poorly understood, but they may minimize secondary structure of plant introns or bind specific UA binding proteins to recruit splicing factors early in spliceosome formation (Simpson and Filipowicz, 1996;Brown and Simpson, 1998;Lorković et al, 2000;Reddy, 2001). Putative UA-rich binding proteins (UBP1, RBP45, and RBP47) with affinity for U-rich sequences have been isolated and characterized (Gniadkowski et al, 1996;Lambermon et al, 2000;Lorković et al, 2002). Of these, only UBP1 so far has been shown to affect splicing, where overexpression of UBP1 enhanced splicing of otherwise poorly spliced U2-type introns .…”

Vascular plant one-zinc-finger protein 2 is localized both to the nucleus and stress granules under heat stress in Arabidopsis

“…UBP1 acts as SGs' central component, and protects heat stress tolerance-related transcripts from exonucleolytic degradation via interactions with the 3 0 -UTR of mRNAs to play a fundamental role in thermotolerance. 13,21,22 In plants, another DNA-and RNA-biding family of proteins, the CCCH-type tandem zinc finger proteins (TZFs), colocalized with both SGs and P-bodies under various stressful conditions including heat and hypoxia. TZFs are highly conserved among eukaryotes and perform critical functions in mRNA metabolism in animals and yeast.…”

Posttranscriptional control of plant development

“…Examples include AtSRp30, a serine/arginine-rich (SR) protein [49], poly(A)-binding proteins [50,51], the U1-70K protein and its interacting SR proteins [52,53], and an exonuclease AtRrp41p [54] from Arabidopsis. Several Nicotiana plumbaginifolia proteins, such as UBP1, a novel hnRNP-like protein [55], the UBP1-interacting proteins UBA1 and UBA2 [56], and RBP45 and RBP47, two oligouridylatespecific hnRNP-like proteins [57], have also been characterized. Although most studies have focused on the expression patterns of genes that encode RNA-binding proteins or on the biochemical activities of the proteins, the developmental roles of some RNA-binding proteins have been explored.…”