A lack of peptide binding and decreased thermostability suggests that the CASKIN2 scaffolding protein SH3 domain may be vestigial.

BACKGROUND: CASKIN2 is a neuronal signaling scaffolding protein comprised of multiple ankyrin repeats, two SAM domains, and one SH3 domain. The CASKIN2 SH3 domain for an NMR structural determination because its peptide-binding cleft appeared to deviate from the repertoire of aromatic enriched amino acids that typically bind polyproline-rich sequences. RESULTS: The structure demonstrated that two non-canonical basic amino acids (K290/R319) in the binding cleft were accommodated well in the SH3 fold. An K290Y/R319W double mutant restoring the typical aromatic amino acids found in the binding cleft resulted in a 20 °C relative increase in the thermal stability. Considering the reduced stability, we speculated that the CASKIN2 SH3 could be a nonfunctional remnant in this scaffolding protein. CONCLUSIONS: While the NMR structure demonstrates that the CASKIN2 SH3 domain is folded, its cleft has suffered two substitutions that prevent it from binding typical polyproline ligands. This observation led us to additionally survey and describe other SH3 domains in the Protein Data Bank that may have similarly lost their ability to promote protein-protein interactions.

A new mode of SAM domain mediated oligomerization observed in the CASKIN2 neuronal scaffolding protein.

BACKGROUND: CASKIN2 is a homolog of CASKIN1, a scaffolding protein that participates in a signaling network with CASK (calcium/calmodulin-dependent serine kinase). Despite a high level of homology between CASKIN2 and CASKIN1, CASKIN2 cannot bind CASK due to the absence of a CASK Interaction Domain and consequently, may have evolved undiscovered structural and functional distinctions. RESULTS: We demonstrate that the crystal structure of the Sterile Alpha Motif (SAM) domain tandem (SAM1-SAM2) oligomer from CASKIN2 is different than CASKIN1, with the minimal repeating unit being a dimer, rather than a monomer. Analytical ultracentrifugation sedimentation velocity methods revealed differences in monomer/dimer equilibria across a range of concentrations and ionic strengths for the wild type CASKIN2 SAM tandem and a structure-directed double mutant that could not oligomerize. Further distinguishing CASKIN2 from CASKIN1, EGFP-tagged SAM tandem proteins expressed in Neuro2a cells produced punctae that were distinct both in shape and size. CONCLUSIONS: This study illustrates a new way in which neuronal SAM domains can assemble into large macromolecular assemblies that might concentrate and amplify synaptic responses.

The solution structures of two prophage homologues of the bacteriophage λ Ea8.5 protein reveal a newly discovered hybrid homeodomain/zinc-finger fold.

A cluster of genes in the exoxis region of bacteriophage λ are capable of inhibiting the initiation of DNA synthesis in Escherichia coli. The most indispensible gene in this region is ea8.5. Here, we report the nuclear magnetic resonance structures of two ea8.5 orthologs from enteropathogenic E. coli and Pseudomonas putida prophages. Both proteins are characterized by a fused homeodomain/zinc-finger fold that escaped detection by primary sequence search methods. While these folds are both associated with a nucleic acid binding function, the amino acid composition suggests otherwise, leading to the possibility that Ea8.5 associates with other viral and host proteins.

HACS1 signaling adaptor protein recognizes a motif in the paired immunoglobulin receptor B cytoplasmic domain.

Hematopoietic adaptor containing SH3 and SAM domains-1 (HACS1) is a signaling protein with two juxtaposed protein-protein interaction domains and an intrinsically unstructured region that spans half the sequence. Here, we describe the interaction between the HACS1 SH3 domain and a sequence near the third immunoreceptor tyrosine-based inhibition motif (ITIM3) of the paired immunoglobulin receptor B (PIRB). From surface plasmon resonance binding assays using a mouse and human PIRB ITIM3 phosphopeptides as ligands, the HACS1 SH3 domain and SHP2 N-terminal SH2 domain demonstrated comparable affinities in the micromolar range. Since the PIRB ITIM3 sequence represents an atypical ligand for an SH3 domain, we determined the NMR structure of the HACS1 SH3 domain and performed a chemical shift mapping study. This study showed that the binding site on the HACS1 SH3 domain for PIRB shares many of the same amino acids found in a canonical binding cleft normally associated with polyproline ligands. Molecular modeling suggests that the respective binding sites in PIRB ITIM3 for the HACS1 SH3 domain and the SHP2 SH2 domain are too close to permit simultaneous binding. As a result, the HACS1-PIRB partnership has the potential to amalgamate signaling pathways that influence both immune and neuronal cell fate.

Essential Gene Profiles for Human Pluripotent Stem Cells Identify Uncharacterized Genes and Substrate Dependencies.

Human pluripotent stem cells (hPSCs) provide an invaluable tool for modeling diseases and hold promise for regenerative medicine. For understanding pluripotency and lineage differentiation mechanisms, a critical first step involves systematically cataloging essential genes (EGs) that are indispensable for hPSC fitness, defined as cell reproduction in this study. To map essential genetic determinants of hPSC fitness, we performed genome-scale loss-of-function screens in an inducible Cas9 H1 hPSC line cultured on feeder cells and laminin to identify EGs. Among these, we found FOXH1 and VENTX, genes that encode transcription factors previously implicated in stem cell biology, as well as an uncharacterized gene, C22orf43/DRICH1. hPSC EGs are substantially different from other human model cell lines, and EGs in hPSCs are highly context dependent with respect to different growth substrates. Our CRISPR screens establish parameters for genome-wide screens in hPSCs, which will facilitate the characterization of unappreciated genetic regulators of hPSC biology.

The NMR structure of the gpU tail-terminator protein from bacteriophage lambda: identification of sites contributing to Mg(II)-mediated oligomerization and biological function.

During the late stages of lambda bacteriophage assembly, the protein gpU terminates tail polymerization and participates at the interface between the mature capsid and tail components. When it engages the lambda tail, gpU undergoes a monomer-hexamer transition to achieve its biologically active form. Towards understanding how gpU participates in multiple protein-protein interactions, we have solved the structure of gpU in its monomeric state using NMR methods. The structure reveals a mixed alpha/beta motif with several dynamic loops at the periphery. Addition of 20 mM MgCl(2) is known to oligomerize gpU in the absence of its protein partners. Multiple image analysis of electron micrographs revealed ring-like structures of magnesium ion saturated gpU with a 30 A pore, consistent with its function as a portal for the passage of viral DNA into the host bacterium. The ability of magnesium ions to promote oligomerization was lost when substitutions were made at a cluster of acidic amino acids in the vicinity of helix alpha2 and the beta1-beta2 loop. Furthermore, substitutions at these sites abolished the biological activity of gpU.

Saccharomyces cerevisiae Ste50 binds the MAPKKK Ste11 through a head-to-tail SAM domain interaction.

In Saccharomyces cerevisiae, signal transduction through pathways governing mating, osmoregulation, and nitrogen starvation depends upon a direct interaction between the sterile alpha motif (SAM) domains of the Ste11 mitogen-activated protein kinase kinase kinase (MAPKKK) and its regulator Ste50. Previously, we solved the NMR structure of the SAM domain from Ste11 and identified two mutants that diminished binding to the Ste50 SAM domain. Building upon the Ste11 study, we present the NMR structure of the monomeric Ste50 SAM domain and a series of mutants bearing substitutions at surface-exposed hydrophobic amino acid residues. The mid-loop (ML) region of Ste11-SAM, defined by helices H3 and H4 and the end-helix (EH) region of Ste50-SAM, defined by helix H5, were sensitive to substitution, indicating that these two surfaces contribute to the high-affinity interaction. The combination of two mutants, Ste11-SAM-L72R and Ste50-SAM-L69R, formed a high-affinity heterodimer unencumbered by competing homotypic interactions that had prevented earlier NMR studies of the wild-type complex. Yeast bearing mutations that prevented the heterotypic Ste11-Ste50 association in vitro presented signaling defects in the mating and high-osmolarity growth pathways.

The NMR and X-ray structures of the Saccharomyces cerevisiae Vts1 SAM domain define a surface for the recognition of RNA hairpins.

The SAM domain of the Saccharomyces cerevisiae post-transcriptional regulator Vts1 has a high affinity towards RNA hairpins containing a CUGGC pentaloop. We present the 1.6 Angstroms X-ray crystal structure of the Vts1 SAM domain in its unliganded state, and the NMR solution structure of this domain in its RNA-bound state. Both structures reveal a canonical five helix SAM domain flanked by additional secondary structural elements at the N and C termini. The two structures are essentially identical, implying that no major structural rearrangements occur upon RNA binding. Amide chemical shift changes map the RNA-binding site to a shallow, basic patch at the junction of helix alpha5 and the loop connecting helices alpha1 and alpha2.

The NMR structure of the murine DLC2 SAM domain reveals a variant fold that is similar to a four-helix bundle.

BACKGROUND: The tumor suppressor DLC2 (Deleted in Liver Cancer -2) participates in cell signaling at the mitochondrial membrane. DLC2 is characterized by a SAM (sterile alpha motif) domain, a Rho GTPase activating protein (GAP) domain, and a START lipid transfer domain. RESULTS: Towards understanding the function of DLC2, we have solved the NMR solution structure of the SAM domain. The DLC2-SAM domain structure reveals an atypical four-helix composition that is distinct from the five-helix SAM domain structures that have been determined to date. From structural alignments, helix 3 of the canonical SAM domain appears to be replaced by shorter, extended secondary structure that follows a similar path. Another difference is demonstrated by helices 1 and 2 that form a helical hairpin that is situated approximately parallel to the canonical helix 5. CONCLUSION: The DLC2-SAM domain adopts a structure that is topologically more similar to an anti-parallel four-helix bundle than a canonical SAM domain. This alternate topology may allow the DLC2-SAM domain to interact with a novel set of ligands.

A Model for Dimerization of the SOX Group E Transcription Factor Family.

Group E members of the SOX transcription factor family include SOX8, SOX9, and SOX10. Preceding the high mobility group (HMG) domain in each of these proteins is a thirty-eight amino acid region that supports the formation of dimers on promoters containing tandemly inverted sites. The purpose of this study was to obtain new structural insights into how the dimerization region functions with the HMG domain. From a mutagenic scan of the dimerization region, the most essential amino acids of the dimerization region were clustered on the hydrophobic face of a single, predicted amphipathic helix. Consistent with our hypothesis that the dimerization region directly contacts the HMG domain, a peptide corresponding to the dimerization region bound a preassembled HMG-DNA complex. Sequence conservation among Group E members served as a basis to identify two surface exposed amino acids in the HMG domain of SOX9 that were necessary for dimerization. These data were combined to make a molecular model that places the dimerization region of one SOX9 protein onto the HMG domain of another SOX9 protein situated at the opposing site of a tandem promoter. The model provides a detailed foundation for assessing the impact of mutations on SOX Group E transcription factors.

The solution structure of the S.cerevisiae Ste11 MAPKKK SAM domain and its partnership with Ste50.

Ste11 is a MAPKKK from Saccharomyces cerevisiae that helps mediate the response to mating pheromone and the ability to thrive in high-salt environments. These diverse functions are facilitated by a direct interaction between the SAM domain of Ste11 with the SAM domain of its regulatory partner, Ste50. We have solved the NMR structure of the Ste11 SAM domain (PDB 1OW5), which reveals a compact, five alpha-helix bundle and a high degree of structural similarity to the Polyhomeotic SAM domain. The combined study of Ste11 SAM rotational correlation times and crosslinking to Ste50-SAM has suggested a mode through which Ste11-SAM oligomerizes and selectively associates with Ste50-SAM. To probe homotypic and heterotypic interations, Ste11-SAM variants each containing a substitution of a surface-exposed hydrophobic residue were constructed. An I59R variant of Ste11-SAM, disrupted binding to Ste50-SAM in vitro. Yeast expressing full-length Ste11-I59R could neither respond to mating pheromone nor thrive in high salt media-demonstrating that the interaction between Ste11 and Ste50 SAM domains is a prerequisite for key signal transduction events.

Solution structure and peptide binding of the PTB domain from the AIDA1 postsynaptic signaling scaffolding protein.

AIDA1 links persistent chemical signaling events occurring at the neuronal synapse with global changes in gene expression. Consistent with its role as a scaffolding protein, AIDA1 is composed of several protein-protein interaction domains. Here we report the NMR structure of the carboxy terminally located phosphotyrosine binding domain (PTB) that is common to all AIDA1 splice variants. A comprehensive survey of peptides identified a consensus sequence around an NxxY motif that is shared by a number of related neuronal signaling proteins. Using peptide arrays and fluorescence based assays, we determined that the AIDA1 PTB domain binds amyloid protein precursor (APP) in a similar manner to the X11/Mint PTB domain, albeit at reduced affinity (∼10 µM) that may allow AIDA1 to effectively sample APP, as well as other protein partners in a variety of cellular contexts.

Solution structure of the carboxy-terminal Tudor domain from human Coilin.

The Cajal body is a dynamic eukaryotic nuclear organelle that is known primarily as an organizational center for the assembly of snRNAs involved in transcript splicing. One of the most critical components of the Cajal body is the scaffolding protein, Coilin. Here, we demonstrate by NMR methods that the carboxy-terminal region contains a Tudor domain. The Tudor domain is atypical due to the presence of several unstructured loops, one greater than thirty amino acids in length. Tudor domains have been noted previously to bind DNA, RNA and modified amino acids. The absence of these sequence and structural signatures in the Coilin Tudor domain supporting these established functions suggests an alternative role.

A nuclear localization signal at the SAM-SAM domain interface of AIDA-1 suggests a requirement for domain uncoupling prior to nuclear import.

The neuronal scaffolding protein AIDA-1 is believed to act as a convener of signals arising at postsynaptic densities. Among the readily identifiable domains in AIDA-1, two closely juxtaposed sterile alpha motif (SAM) domains and a phosphotyrosine binding domain are located within the C-terminus of the longest splice variant and exclusively in four shorter splice variants. As a first step towards understanding the possible emergent properties arising from this assembly of ligand binding domains, we have used NMR methods to solve the first structure of a SAM domain tandem. Separated by a 15-aa linker, the two SAM domains are fused in a head-to-tail orientation that has been observed in other hetero- and homotypic SAM domain structures. The basic nuclear import signal for AIDA-1 is buried at the interface between the two SAM domains. An observed disparity between the thermal stabilities of the two SAM domains suggests a mechanism whereby the second SAM domain decouples from the first SAM domain to facilitate translocation of AIDA-1 to the nucleus.