Molecular basis of the interaction of Saccharomyces cerevisiae Eaf3 chromo domain with methylated H3K36

Eaf3 is a component of both NuA4 histone acetyltransferase and Rpd3S histone deacetylase complexes in Saccharomyces cerevisiae. It is involved in the regulation of the global pattern of histone acetylation that distinguishes promoters from coding regions. Eaf3 contains a chromo domain at the N terminus that can bind to methylated Lys-36 of histone H3 (H3K36). We report here the crystal structures of the Eaf3 chromo domain in two truncation forms. Unlike the typical HP1 and Polycomb chromo domains, which contain a large groove to bind the modified histone tail, the Eaf3 chromo domain assumes an autoinhibited chromo barrel domain similar to the human MRG15 chromo domain. Compared with other chromo domains, the Eaf3 chromo domain contains a unique 38-residue insertion that folds into two short beta-strands and a long flexible loop to flank the beta-barrel core. Both isothermal titration calorimetry and surface plasmon resonance studies indicate that the interaction between the Eaf3 chromo domain and the trimethylated H3K36 peptide is relatively weak, with a K(D) of approximately 10(-4) m. NMR titration studies demonstrate that the methylated H3K36 peptide is bound to the cleft formed by the C-terminal alpha-helix and the beta-barrel core. Site-directed mutagenesis study and in vitro binding assay results show that the conserved aromatic residues Tyr-23, Tyr-81, Trp-84, and Trp-88, which form a hydrophobic pocket at one end of the beta-barrel, are essential for the binding of the methylated H3K36. These results reveal the molecular mechanism of the recognition and binding of the methylated H3K36 by Eaf3 and provide new insights into the functional roles of the Eaf3 chromo domain.     

Structural basis of HP1/PXVXL motif peptide interactions and HP1 localisation to heterochromatin

HP1 family proteins are adaptor molecules, containing two related chromo domains that are required for chromatin packaging and gene silencing. Here we present the structure of the chromo shadow domain from mouse HP1beta bound to a peptide containing a consensus PXVXL motif found in many HP1 binding partners. The shadow domain exhibits a novel mode of peptide recognition, where the peptide binds across the dimer interface, sandwiched in a beta-sheet between strands from each monomer. The structure allows us to predict which other shadow domains bind similar PXVXL motif-containing peptides and provides a framework for predicting the sequence specificity of the others. We show that targeting of HP1beta to heterochromatin requires shadow domain interactions with PXVXL-containing proteins in addition to chromo domain recognition of Lys-9-methylated histone H3. Interestingly, it also appears to require the simultaneous recognition of two Lys-9-methylated histone H3 molecules. This finding implies a further complexity to the histone code for regulation of chromatin structure and suggests how binding of HP1 family proteins may lead to its condensation.     

Structural comparisons of TIM barrel proteins suggest functional and evolutionary relationships between beta-galactosidase and other glycohydrolases

Beta-galactosidase (lacZ) from Escherichia coli is a 464 kDa homotetramer. Each subunit consists of five domains, the third being an alpha/beta barrel that contains most of the active site residues. A comparison is made between each of the domains and a large set of proteins representative of all structures from the protein data bank. Many structures include an alpha/beta barrel. Those that are most similar to the alpha/beta barrel of E. coli beta-galactosidase have similar catalytic residues and belong to the so-called "4/7 superfamily" of glycosyl hydrolases. The structure comparison suggests that beta-amylase should also be included in this family. Of three structure comparison methods tested, the "ProSup" procedure of Zu-Kang and Sippl and the "Superimpose" procedure of Diederichs were slightly superior in discriminating the members of this superfamily, although all procedures were very powerful in identifying related protein structures. Domains 1, 2, and 4 of E. coli beta-galactosidase have topologies related to "jelly-roll barrels" and "immunoglobulin constant" domains. This fold also occurs in the cellulose binding domains (CBDs) of a number of glycosyl hydrolases. The fold of domain 1 of E. coli beta-galactosidase is closely related to some CBDs, and the domain contributes to substrate binding, but in a manner unrelated to cellulose binding by the CBDs. This is typical of domains 1, 2, 4, and 5, which appear to have been recruited to play roles in beta-galactosidase that are unrelated to the functions that such domains provide in other contexts. It is proposed that beta-galactosidase arose from a prototypical single domain alpha/beta barrel with an extended active site cleft. The subsequent incorporation of elements from other domains could then have reduced the size of the active site from a cleft to a pocket to better hydrolyze the disaccharide lactose and, at the same time, to facilitate the production of inducer, allolactose.     

Molecular biology of the chromo domain: an ancient chromatin module comes of age

The chromo domain motif is found in proteins from fungi, protists, plants, fish, insects, amphibians, birds, and mammals. The chromo domain peptide fold may have its origins as a chromosomal protein in a common ancestor of archea and eukaryota, making it a particularly ancient protein structural module. Chromo domains have been found in single or multiple copies in proteins with diverse structures and activities, most or all of which are connected with chromosome structure/function. In this review, our current knowledge of chromo domain properties is summarized and a variety of contexts in which chromo domains participate in aspects of chromatin metabolism are discussed.     

Tandem histone folds in the structure of the N-terminal segment of the ras activator Son of Sevenless

The Ras activator Son of Sevenless (Sos) contains a Cdc25 homology domain, responsible for nucleotide exchange, as well as Dbl/Pleckstrin homology (DH/PH) domains. We have determined the crystal structure of the N-terminal segment of human Sos1 (residues 1-191) and show that it contains two tandem histone folds. While the N-terminal domain is monomeric in solution, its structure is surprisingly similar to that of histone dimers, with both subunits of the histone "dimer" being part of the same peptide chain. One histone fold corresponds to the region of Sos that is clearly similar in sequence to histones (residues 91-191), whereas the other is formed by residues in Sos (1-90) that are unrelated in sequence to histones. Residues that form a contiguous patch on the surface of the histone domain of Sos are conserved from C. elegans to humans, suggesting a potential role for this domain in protein-protein interactions.     

Crystal structure of RumA, an iron-sulfur cluster containing E. coli ribosomal RNA 5-methyluridine methyltransferase

RumA catalyzes transfer of a methyl group from S-adenosylmethionine (SAM) specifically to uridine 1939 of 23S ribosomal RNA in Escherichia coli to yield 5-methyluridine. We determined the crystal structure of RumA at 1.95 A resolution. The protein is organized into three structural domains: The N-terminal domain contains sequence homology to the conserved TRAM motif and displays a five-stranded beta barrel architecture characteristic of an oligosaccharide/oligonucleotide binding fold. The central domain contains a [Fe(4)S(4)] cluster coordinated by four conserved cysteine residues. The C-terminal domain displays the typical SAM-dependent methyltransferase fold. The catalytic nucleophile Cys389 lies in a motif different from that in DNA 5-methylcytosine methyltransferases. The electrostatic potential surface reveals a predominately positively charged area that covers the concave surface of the first two domains and suggests an RNA binding mode. The iron-sulfur cluster may be involved in the correct folding of the protein or may have a role in RNA binding.     

The HP1 chromo shadow domain binds a consensus peptide pentamer

Heterochromatin-associated protein 1 (HP1) is thought to affect chromatin structure through interactions with other proteins in heterochromatin. Chromo domains located near the amino (amino chromo) and carboxy (chromo shadow) termini of HP1 may mediate such interactions, as suggested by domain swapping, in vitro binding and 3D structural studies . Several HP1-associated proteins have been reported, providing candidates that might specifically complex with the chromo domains of HP1. However, such association studies provide little mechanistic insight and explore only a limited set of potential interactions in a largely non-competitive setting. To determine how chromo domains can selectively interact with other proteins, we probed random peptide phage display libraries using chromo domains from HP1. Our results demonstrate that a consensus pentapeptide is suffident for specific interaction with the HP1 chromo shadow domain. The pentapeptide is found in the amino acid sequence of reported HP1-associated proteins, including the shadow domain itself. Peptides that bind the shadow domain also disrupt shadow domain dimers. Our results suggest that HP1 dimerization, which is thought to mediate heterochromatin compaction and cohesion, occurs via pentapeptide binding. In general, chromo domains may function by avidly binding short peptides at the surface of chromatin-associated proteins.     

The structure of mouse HP1 suggests a unique mode of single peptide recognition by the shadow chromo domain dimer

The heterochromatin protein 1 (HP1) family of proteins is involved in gene silencing via the formation of heterochromatic structures. They are composed of two related domains: an N-terminal chromo domain and a C-terminal shadow chromo domain. Present results suggest that chromo domains may function as protein interaction motifs, bringing together different proteins in multi-protein complexes and locating them in heterochromatin. We have previously determined the structure of the chromo domain from the mouse HP1beta protein, MOD1. We show here that, in contrast to the chromo domain, the shadow chromo domain is a homodimer. The intact HP1beta protein is also dimeric, where the interaction is mediated by the shadow chromo domain, with the chromo domains moving independently of each other at the end of flexible linkers. Mapping studies, with fragments of the CAF1 and TIF1beta proteins, show that an intact, dimeric, shadow chromo domain structure is required for complex formation.     

The Solution Structure of the C-Terminal Ig-like Domain of the Bacteriophage λ Tail Tube Protein

Immunoglobulin (Ig)-like domains are found frequently on the surface of tailed double-stranded DNA bacteriophages, yet their functional role remains obscure. Here, we have investigated the structure and function of the C-terminal Ig-like domain of gpV (gpV_C), the tail tube protein of phage λ. This domain has been predicted through sequence similarity to be a member of the bacterial Ig-like domain 2 (Big_2) family, which is composed of more than 1300 phage and bacterial sequences. Using trypsin proteolysis, we have delineated the boundaries of gpV_C and have shown that its removal reduces the biological activity of gpV by 100-fold; thus providing a definitive demonstration of a functional role for this domain. Determination of the solution structure of gpV_C by NMR spectroscopy showed that it adopts a canonical Ig-like fold of the I-set class. This represents the first structure of a phage-encoded Ig-like domain and only the second structure of a Big_2 domain. Structural and sequence comparisons indicate that the gpV_C structure is more representative of both the phage-encoded Big_2 domains and Big_2 domains in general than the other available Big_2 structure. Bioinformatics analyses have identified two conserved clusters of residues on the surface of gpV_C that may be important in mediating the function of this domain.     

Structure of the enabled/VASP homology 1 domain-peptide complex: a key component in the spatial control of actin assembly.

The Enabled/VASP homology 1 (EVH1; also called WH1) domain is an interaction module found in several proteins implicated in actin-based cell motility. EVH1 domains bind the consensus proline-rich motif FPPPP and are required for targeting the actin assembly machinery to sites of cytoskeletal remodeling. The crystal structure of the mammalian Enabled (Mena) EVH1 domain complexed with a peptide ligand reveals a mechanism of recognition distinct from that used by other proline-binding modules. The EVH1 domain fold is unexpectedly similar to that of the pleckstrin homology domain, a membrane localization module. This finding demonstrates the functional plasticity of the pleckstrin homology fold as a binding scaffold and suggests that membrane association may play an auxiliary role in EVH1 targeting.     

Structure and function insights garnered from in silico modeling of the thrombospondin type-1 domain-containing 7A antigen

The thrombospondin type-1 domain containing 7A (THSD7A) protein is known to be one of the antigens responsible for the autoimmune disorder idiopathic membranous nephropathy. The structure of this antigen is currently unsolved experimentally. Here we present a homology model of the extracellular portion of the THSD7A antigen. The structure was evaluated for folding patterns, epitope site prediction, and function was predicted. Results show that this protein contains 21 extracellular domains and with the exception of the first two domains, has a regular repeating pattern of TSP-1-like followed by F-spondin-like domains. Our results indicate the presence of a novel Trp-ladder sequence of WxxxxW in the TSP-1-like domains. Of the 21 domains, 18 were shown to have epitope binding sites as predicted by epitopia. Several of the F-spondin-like domains have insertions in the canonical TSP fold, most notably the coiled coil region in domain 4, which may be utilized in protein-protein binding interactions, suggesting that this protein functions as a heparan sulfate binding site.     

Amino-terminal dimerization, NRDP1-rhodanese interaction, and inhibited catalytic domain conformation of the ubiquitin-specific protease 8 (USP8)

Ubiquitin-specific protease 8 (USP8) hydrolyzes mono and polyubiquitylated targets such as epidermal growth factor receptors and is involved in clathrin-mediated internalization. In 1182 residues, USP8 contains multiple domains, including coiled-coil, rhodanese, and catalytic domains. We report the first high-resolution crystal structures of these domains and discuss their implications for USP8 function. The amino-terminal domain is a homodimer with a novel fold. It is composed of two five-helix bundles, where the first helices are swapped, and carboxyl-terminal helices are extended in an antiparallel fashion. The structure of the rhodanese domain, determined in complex with the E3 ligase NRDP1, reveals the canonical rhodanese fold but with a distorted primordial active site. The USP8 recognition domain of NRDP1 has a novel protein fold that interacts with a conserved peptide loop of the rhodanese domain. A consensus sequence of this loop is found in other NRDP1 targets, suggesting a common mode of interaction. The structure of the carboxyl-terminal catalytic domain of USP8 exhibits the conserved tripartite architecture but shows unique traits. Notably, the active site, including the ubiquitin binding pocket, is in a closed conformation, incompatible with substrate binding. The presence of a zinc ribbon subdomain near the ubiquitin binding site further suggests a polyubiquitin-specific binding site and a mechanism for substrate induced conformational changes.