Human hnRNP A1 is a versatile single-stranded nucleic acid-binding protein that functions in various aspects of mRNA maturation and in telomere length regulation. The crystal structure of UP1, the amino-terminal domain of human hnRNP A1 containing two RNA-recognition motifs (RRMs), bound to a 12-nucleotide single-stranded telomeric DNA has been determined at 2.1 Å resolution. The structure of the complex reveals the basis for sequence-specific recognition of the single-stranded overhangs of human telomeres by hnRNP A1. It also provides insights into the basis for high-affinity binding of hnRNP A1 to certain RNA sequences, and for nucleic acid binding and functional synergy between the RRMs. In the crystal structure, a UP1 dimer binds to two strands of DNA, and each strand contacts RRM1 of one monomer and RRM2 of the other. The two DNA strands are antiparallel, and regions of the protein flanking each RRM make important contacts with DNA. The extensive protein-protein interface seen in the crystal structure of the protein-DNA complex and the evolutionary conservation of the interface residues suggest the importance of specific protein-protein interactions for the sequence-specific recognition of single-stranded nucleic acids. Models for regular packaging of telomere 3 overhangs and for juxtaposition of alternative 5 splice sites are proposed.[Key Words: hnRNP A1; telomere; RNA-recognition motif; x-ray crystallography]Received February 11, 1999; revised version accepted March 9, 1999.Heterogeneous nuclear ribonucleoprotein (hnRNP) A1 is one of the most abundant and best-studied components of hnRNP complexes. Together with other hnRNP proteins, hnRNP A1 packages nascent pre-messenger RNA (pre-mRNA) for processing in the nucleus (for review, see Dreyfuss et al. 1993;McAfee et al. 1997). Although it accumulates predominantly in the nucleus, hnRNP A1 shuttles continuously between the nucleus and the cytoplasm (Piñ ol-Roma and Dreyfuss 1992). Nuclear localization and import-export signals within hnRNP A1 have been mapped to a 38 amino acid sequence, known as M9, located near the carboxyl terminus of the protein (for review, see Izaurralde and Adam 1998). Because hnRNP A1 associates with poly(A) + RNA both in the nucleus and in the cytoplasm, and injection of hnRNP A1 with an intact M9 region into frog oocytes inhibits mRNA export, it is likely that hnRNP A1 is involved in transporting mature mRNA from the nucleus to the cytoplasm.Nuclear hnRNP A1 and other closely related hnRNP A/B proteins can modulate the use of alternative 5Ј splice sites in a concentration-dependent manner. hnRNP A1 activates distal 5Ј splice sites and promotes alternative exon skipping both in vitro and in vivo (Fu et al. 1992;Mayeda and Krainer 1992;Mayeda et al. 1993;Cá ceres et al. 1994;Yang et al. 1994). The alternative splicing activity of hnRNP A1 counteracts that of members of the serine-arginine (SR) family of splicing factors, such as SF2/ASF and SC35. The relative levels or activities of these two classes of antagonistic factors can dete...

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SUMMARY The postsynaptic density (PSD) is crucial for synaptic functions, but the molecular architecture retaining its structure and components remains elusive. Homer and Shank are among the most abundant scaffolding proteins in the PSD, working synergistically for maturation of dendritic spines. Here, we demonstrate that Homer and Shank, together, form a mesh-like matrix structure. Crystallographic analysis of this region revealed a pair of parallel dimeric coiled-coils intercalated in a tail-to-tail fashion to form a tetramer, giving rise to the unique configuration of a pair of amino-terminal EVH1 domains at each end of the coiled-coil. In neurons, the tetramerization is required for structural integrity of the dendritic spines and recruitment of proteins to synapses. We propose that the Homer-Shank complex serves as a structural framework and as an assembly platform for other PSD proteins.

contributed equally to this workThe N-terminal domain of the largest subunit of the Saccharomyces cerevisiae origin recognition complex (Orc1p) functions in transcriptional silencing and contains a bromo-adjacent homology (BAH) domain found in some chromatin-associated proteins including Sir3p. The 2.2 A Ê crystal structure of the N-terminal domain of Orc1p revealed a BAH core and a non-conserved helical sub-domain. Mutational analyses demonstrated that the helical sub-domain was necessary and suf®cient to bind Sir1p, and critical for targeting Sir1p primarily to the cis-acting E silencers at the HMR and HML silent chromatin domains. In the absence of the BAH domain,~14±20% of cells in a population were silenced at the HML locus. Moreover, the distributions of the Sir2p, Sir3p and Sir4p proteins, while normal, were at levels lower than found in wild-type cells. Thus, in the absence of the Orc1p BAH domain, HML resembled silencing of genes adjacent to telomeres. These data are consistent with the view that the Orc1p±Sir1p interaction at the E silencers ensures stable inheritance of pre-established Sir2p, Sir3p and Sir4p complexes at the silent mating type loci.