Peter R. Nielsen

Structure of the HP1 chromodomain bound to histone H3 methylated at lysine 9

Specific modifications to histones are essential epigenetic markers—heritable changes in gene expression that do not affect the DNA sequence. Methylation of lysine 9 in histone H3 is recognized by heterochromatin protein 1 (HP1), which directs the binding of other proteins to control chromatin structure and gene expression. Here we show that HP1 uses an induced-fit mechanism for recognition of this modification, as revealed by the structure of its chromodomain bound to a histone H3 peptide dimethylated at Nζ of lysine 9. The binding pocket for the N-methyl groups is provided by three aromatic side chains, Tyr 21, Trp 42 and Phe 45, which reside in two regions that become ordered on binding of the peptide. The side chain of Lys 9 is almost fully extended and surrounded by residues that are conserved in many other chromodomains. The QTAR peptide sequence preceding Lys 9 makes most of the additional interactions with the chromodomain, with HP1 residues Val 23, Leu 40, Trp 42, Leu 58 and Cys 60 appearing to be a major determinant of specificity by binding the key buried Ala 7. These findings predict which other chromodomains will bind methylated proteins and suggest a motif that they recognize.

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 HP1β protein, MOD1. We show here that, in contrast to the chromo domain, the shadow chromo domain is a homodimer. The intact HP1β 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 TIF1β proteins, show that an intact, dimeric, shadow chromo domain structure is required for complex formation.

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 HP1β 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 β‐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 HP1β 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.

Structure of the C-terminal Domain from Trypanosoma brucei Variant Surface Glycoprotein MITat1.2

The variant surface glycoprotein (VSG) of African trypanosomes has a structural role in protecting other cell surface proteins from effector molecules of the mammalian immune system and also undergoes antigenic variation necessary for a persistent infection in a host. Here we have reported the solution structure of a VSG type 2 C-terminal domain from MITat1.2, completing the first structure of both domains of a VSG. The isolated C-terminal domain is a monomer in solution and forms a novel fold, which commences with a short α-helix followed by a single turn of 310-helix and connected by a short loop to a small anti-parallel β-sheet and then a longer α-helix at the C terminus. This compact domain is flanked by two unstructured regions. The structured part of the domain contains 42 residues, and the core comprises 2 disulfide bonds and 2 hydrophobic residues. These cysteines and hydrophobic residues are conserved in other VSGs, and we have modeled the structures of two further VSG C-terminal domains using the structure of MITat1.2. The models suggest that the overall structure of the core is conserved in the different VSGs but that the C-terminal α-helix is of variable length and depends on the presence of charged residues. The results provided evidence for a conserved tertiary structure for all the type 2 VSG C-terminal domains, indicated that VSG dimers form through interactions between N-terminal domains, and showed that the selection pressure for sequence variation within a conserved tertiary structure acts on the whole of the VSG molecule.

Structure of the Sterile α Motif (SAM) Domain of the Saccharomyces cerevisiae Mitogen-activated Protein Kinase Pathway-modulating Protein STE50 and Analysis of Its Interaction with the STE11 SAM

The sterile α motif (SAM) is a 65-70-amino acid domain found in over 300 proteins that are involved in either signal transduction or transcriptional activation and repression. SAM domains have been shown to mediate both homodimerization and heterodimerization and in some cases oligomerization. Here, we present the solution structure of the SAM domain of the Saccharomyces cerevisiae protein, Ste50p. Ste50p functions as a modulator of the mitogen-activated protein kinase (MAPK) cascades in S. cerevisiae, which control mating, pseudohyphal growth, and osmo-tolerance. This is the first example of the structure of a SAM domain from a MAPK module protein. We have studied the associative behavior of Ste50p SAM in solution and shown that it is monomeric. We have examined the SAM domain from Ste11p, the MAPK kinase kinase that associates with Ste50p in vivo, and shown that it forms dimers with a self-association Kd of ∼0.5 mm. We have also analyzed the interaction of Ste50p SAM with Ste11p SAM and the effects of mutations at Val-37, Asp-38, Pro-71, Leu-73, Leu-75, and Met-99 of STE50 on the heterodimerization properties of Ste50p SAM. We have found that L73A and L75A abrogate the Ste50p interaction with Ste11p, and we compare these data with the known interaction sites defined for other SAM domain interactions.

Thermodynamic characterization of the palm tree Roystonea regia peroxidase stability.

The structural stability of a peroxidase, a dimeric protein from royal palm tree (Roystonea regia) leaves, has been characterized by high-sensitivity differential scanning calorimetry, circular dichroism, steady-state tryptophan fluorescence and analytical ultracentifugation under different solvent conditions. It is shown that the thermal and chemical (using guanidine hydrochloride (Gdn-HCl)) folding/unfolding of royal palm tree peroxidase (RPTP) at pH 7 is a reversible process involving a highly cooperative transition between the folded dimer and unfolded monomers, with a free stabilization energy of about 23 kcal per mol of monomer at 25 degrees C. The structural stability of RPTP is pH-dependent. At pH 3, where ion pairs have disappeared due to protonation, the thermally induced denaturation of RPTP is irreversible and strongly dependent upon the scan rate, suggesting that this process is under kinetic control. Moreover, thermally induced transitions at this pH value are dependent on the protein concentration, allowing it to be concluded that in solution RPTP behaves as dimer, which undergoes thermal denaturation coupled with dissociation. Analysis of the kinetic parameters of RPTP denaturation at pH 3 was accomplished on the basis of the simple kinetic scheme N-->kD, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state, and thermodynamic information was obtained by extrapolation of the kinetic transition parameters to an infinite heating rate. Obtained in this way, the value of RPTP stability at 25 degrees C is ca. 8 kcal per mole of monomer lower than at pH 7. In all probability, this quantity reflects the contribution of ion pair interactions to the structural stability of RPTP. From a comparison of the stability of RPTP with other plant peroxidases it is proposed that one of the main factors responsible for the unusually high stability of RPTP which enhances its potential use for biotechnological purposes, is its dimerization.

Structure determination of protein complexes by NMR.

This chapter describes nuclear magnetic resonance (NMR) methods that can be used to determine the structures of protein complexes. Many of these techniques are also applicable to other systems (e.g., protein-nucleic acid complexes). In the first section, we discuss methodologies for optimizing the sample conditions for the study of complexes. This is followed by a description of the methods that can be used to map interfaces when a full structure determination of the complex is not appropriate or not possible. We then describe experimental approaches for resonance assignment in complexes, these are essentially the same as those for isolated proteins. Subheading 6. describes the different types of so-called X-filtered NMR experiments that have been devised to separate and selectively observe either inter- or intramolecular structural information. These filtered NMR experiments are then exploited in the experimental strategies for structure determination of either protein complexes or homodimeric proteins. This is followed by a description of the calculation of their structures. Finally, we present case studies from three projects carried out in our laboratory, where we successfully used the methods presented in this chapter.

Expression, purification, and biophysical studies of chromodomain proteins

Publisher Summary This chapter describes the structural features of chromodomains and mentions how this information can be used to identify chromodomains from amino acid sequences. The chapter employs various biophysical techniques to study the chromodomain and chromo shadow domains of HP1β. Chromodomains are one of a number of similar domains that are involved in chromatin structure. Chromodomains can be divided into two subfamilies—namely, the chromodomains and the chromo shadow domains. Fluorescence spectroscopy is an ideal method to study the interaction of the HP1β chromodomain with histone H3. The interaction is mediated via the methyl groups of lysine 9 of histone H3, which fit into a hydrophobic pocket on the surface of the chromodomain formed by the aromatic residues Tyr 21, Phe 45, and Trp 42. Protein fluorescence originates from the aromatic residues, Phe, Tyr, and Trp. However, the fluorescence of proteins containing all three aromatic amino acids is usually dominated by tryptophan. In addition, the chapter also describes how proteins can be characterized and purified using steady-state fluorescence spectroscopy, analytical ultracentrifugation (AUC), and nuclear magnetic resonance (NMR) spectroscopy.