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An essential yeast protein, Nup145p, is identified via its genetic interaction with the nucleoporin Nsp1p. Nup145p contains GLFG repeats and localizes to nuclear pores. Depletion of Nup145p in vivo leads rapidly to nuclear retention of polyadenylated RNAs and more slowly to cytoplasmic accumulation of a nuclear reporter protein. A stretch of 140 amino acids within Nup145p is conserved in two other yeast nucleoporins, Nup116p and Nup100p, and in an uncharacterized C. elegans protein. Genetic experiments in yeast reveal that the three copies of the motif carry out an essential, redundant function. Fragments of Nup145p and Nup116p including this motif bind specifically to homopolymeric RNAs in vitro. Nup145p, Nup116p, and Nup100p thus represent a novel class of nucleoporins involved in nucleocytoplasmic transport.

The human U1 snRNP-specific U1A protein autoregulates its production by binding its own pre-mRNA and inhibiting polyadenylation. The mechanism of this regulation has been elucidated by in vitro studies. U1A protein is shown not to prevent either binding of cleavage and polyadenylation specificity factor (CPSF) to its recognition sequence (AUUAAA) or to prevent cleavage of U1A pre-mRNA. Instead, U1A protein bound to U1A pre-mRNA inhibits both specific and nonspecific polyadenylation by mammalian, but not by yeast, poly(A) polymerase (PAP). Domains are identified in both proteins whose removal uncouples the polyadenylation activity of mammalian PAP from its inhibition via RNA-bound U1A protein. Finally, U1A protein is shown to specifically interact with mammalian PAP in vitro. The possibility that this interaction may reflect a broader role of the U1A protein in polyadenylation is discussed.

Human, mouse, and Xenopus mRNAs encoding the U1 snRNP-specific U1 A protein contain a conserved 47 nt region in their 3′ untranslated regions (UTRs). In vitro studies show that human U1A protein binds to two sites within the conserved region that resemble, in part, the previously characterized UlA-binding site on U1 snRNA. Overexpression of human U1A protein in mouse cells results in down-regulation of endogenous mouse UlA mRNA accumulation. In vitro and in vivo experiments demonstrate that excess UIA protein specifically inhibits polyadenylation of pre-mRNAs that contain the conserved 3′ UTR from human UlA mRNA. Thus, U1 A protein regulates the production of its own mRNA via a mechanism that involves pre-mRNA binding and inhibition of polyadenylation.

A number of mink cell focus-forming (MCF) proviruses was molecularly cloned from mouse lymphoma DNA. From each clone, flanking probes were prepared to detect common integration regions in other MuLV-induced lymphomas. One clone frequently revealed variations in the molecular structure of the corresponding region (Pim-1) in other lymphomas. The results show the following. Changes in the Pim region are seen in 24 out of 93 lymphomas tested. Over 50% of the early T-cell lymphomas show integration in the Pim-1 region. The alterations are seen in different mouse strains and with various MuLVs. The observed variations are caused by the integration of predominantly MCF genomes. All integrations occur in a region spanning less than 20 kb and are associated with the transcriptional activation of a distinct region within the Pim-1 domain. The activated region does not show any homology with 13 known and three putative oncogenes.

HIV-1 Rev protein directs nuclear export of pre-mRNAs and mRNAs containing its binding site, the Rev response element (RRE). To define how Rev acts, we used conjugates between bovine serum albumin (BSA) and peptides comprising the Rev activation domain (BSA-R). BSA-R inhibited Rev-mediated nuclear RNA export, whereas a mutant activation domain peptide conjugate did not. BSA-R did not affect the export of mRNA, tRNA, or ribosomal subunits, but did inhibit export of 5S rRNA and spliceosomal U snRNAs. BSA-R was itself exported from the nucleus in an active, saturable manner. Thus, the Rev activation domain constitutes a nuclear export signal that redirects RRE-containing ciral RNAs to a non-mRNA export pathway.

We have shown previously that the putative oncogene pim-1 is frequently activated by provirus insertion in murine leukemia virus-induced T cell lymphomas. Here we describe the structure of the pim-1 gene as determined by sequencing genomic and cDNA clones. The gene has an open reading frame, encoding a protein of 313 amino acids, extending over six exons and preceded and followed by stop codons in all reading frames. Proviruses always integrate outside the protein-encoding domain, showing a high preference for a small region in the 3'-terminal exon; integration in the 3' exon results in relatively high levels of pim-1 mRNA. Computer search reveals homology between pim-1 and protein kinases: all the domains characteristic of protein kinases are conserved in the pim-1 amino acid sequence. The highest homologies were observed with the protein-serine kinases.