The RNA‐binding protein ALYREF plays key roles in nuclear export and also 3′‐end processing of polyadenylated mRNAs, but whether such regulation also extends to non‐polyadenylated RNAs is unknown. Replication‐dependent (RD)‐histone mRNAs are not polyadenylated, but instead end in a stem‐loop (SL) structure. Here, we demonstrate that ALYREF prevalently binds a region next to the SL on RD‐histone mRNAs. SL‐binding protein (SLBP) directly interacts with ALYREF and promotes its recruitment. ALYREF promotes histone pre‐mRNA 3′‐end processing by facilitating U7‐snRNP recruitment through physical interaction with the U7‐snRNP‐specific component Lsm11. Furthermore, ALYREF, together with other components of the TREX complex, enhances histone mRNA export. Moreover, we show that 3′‐end processing promotes ALYREF recruitment and histone mRNA export. Together, our results point to an important role of ALYREF in coordinating 3′‐end processing and nuclear export of non‐polyadenylated mRNAs.

A successful pregnancy depends on the blastocyst's implantation to the maternal endometrium; however, the initial interaction between blastocyst and uterine epithelium has not been well characterized. The objectives of this study were to determine if selectins and their ligands were expressed in the bovine conceptus and/or uterus during the periattachment period and to study whether selectins were associated with conceptus attachment to the uterine epithelium. Through the RNA-sequence analysis of bovine conceptuses on Days 17, 20, and 22 (Day 0 = day of estrus), only the SELL ligand, podocalyxin (PODXL), and P-selectin (SELP) ligand, SELPLG, were found. Quantitative PCR analysis confirmed the presence of PODXL and SELPLG in these conceptuses and revealed that SELL, mRNA and protein, detected in the uterine epithelium but not in conceptuses increased during the periattachment period. In the cultured endometrial epithelial cells (EECs), SELL transcript was up-regulated when uterine flushings from Day 20 pregnant animals were placed onto these cells. SELL was also up-regulated when cultured EECs were treated with progesterone, EGF, or bFGF, but not with IFNT. In the coculture system with EECs and bovine trophoblast CT-1 cells, SELL expression in EECs was effectively reduced by its small interfering RNA; however, IFNT, a marker for CT-1 cell attachment to EECs, was not reduced, nor was a transcription factor of IFNT, CDX2. These observations suggest that the conceptus could attach to the uterine epithelium through the use of endometrial SELL and embryonic selectin ligands, possibly initiating the conceptus attachment process in the bovine species.

To ensure efficient and accurate gene expression, pre-mRNA processing and mRNA export need to be balanced. However, how this balance is ensured remains largely unclear. Here, we found that SF3b, a component of U2 snRNP that participates in splicing and 3′ processing of pre-mRNAs, interacts with the key mRNA export adaptor THO in vivo and in vitro. Depletion of SF3b reduces THO binding with the mRNA and causes nuclear mRNA retention. Consistently, introducing SF3b binding sites into the mRNA enhances THO recruitment and nuclear export in a dose-dependent manner. These data demonstrate a role of SF3b in promoting mRNA export. In support of this role, SF3b binds with mature mRNAs in the cells. Intriguingly, disruption of U2 snRNP by using a U2 antisense morpholino oligonucleotide does not inhibit, but promotes, the role of SF3b in mRNA export as a result of enhanced SF3b-THO interaction and THO recruitment to the mRNA. Together, our study uncovers a U2-snRNPindependent role of SF3b in mRNA export and suggests that SF3b contributes to balancing pre-mRNA processing and mRNA export.SF3b | THO | mRNA export | pre-mRNA processing | U2 snRNP I n eukaryotes, the nascent pre-mRNA transcripts undergo multiple processing steps in the nucleus before mRNAs are exported to the cytoplasm for translation. Accumulating evidence suggests that pre-mRNA processing and mRNA export need to be balanced to ensure efficient and accurate gene expression (1-7). When splicing factors are limited or when mRNA export factors are present in excess, even unspliced pre-mRNAs are leaked to the cytoplasm (1-3). On the contrary, down-regulation of nuclear export factors results in nuclear retention of fully processed mRNAs that are ultimately subject to degradation (4-7). Thus, maintenance of the balance between pre-mRNA processing and mRNA export is of significant importance. However, how this balance is achieved remains largely unknown.U2 snRNP is a core component of the spliceosome. It is comprised of U2 snRNA, multisubunit SF3a and SF3b complexes, U2-snRNPspecific proteins A′ and B″, as well as the seven Sm proteins common to the spliceosomal snRNPs (8-10). During splicing, SF3b proteins contact the pre-mRNA at and near the branch site (BS) in a sequence-independent manner, stabilizing the U2 snRNA/BS interaction (11-18), and thereby regulate BS recognition and selection. Except for splicing, U2 snRNP also functions in the 3′ processing of polyadenylated and nonpolyadenylated mRNAs (19,20). On polyadenylated pre-mRNAs, U2 snRNP components including SF3b interact with the cleavage and polyadenylation specificity factor and enhance the rate of 3′ processing (19). On nonpolyadenylated histone pre-mRNAs, SF3b155, the largest SF3b subunit, together with Prp43, directly makes contact with the 7-nt motif, C/GAAGAAG, present in the coding region and facilitates 3′ processing as a component of U2 snRNP (20). To date, all known roles of SF3b are executed in the context of U2 snRNP, and whether it has a U2-snRNP-independent role remains unknown.The highly c...