Novel Functional Features of the LIS-H Domain: Role in Protein Dimerization,Half-Life and Cellular Localization

The presence of a conserved protein motif usually implies common functional features. Here, we focused on the LisH (LIS1 homology) domain, which is found in multiple proteins, and have focused on three involved in human genetic diseases; LIS1, Transducin beta-like 1X (TBL1) and Oral-facial-digital type 1 (OFD1). The recently solved structure of the LisH domain in the N-terminal region of LIS1 depicted it as a novel dimerization motif. Our findings indicated that the LisH domain of both LIS1 and TBL1 is essential for in vitro oligomerization. Furthermore, our study disclosed novel in vivo features of the LisH motif. Mutationsin conserved LisH amino acids significantly reduced both the protein half-life of LIS1, TBL1, and OFD1, and dramatically affected specific intracellular localizations of these proteins. LIS1 mutated in the LisH domain induced its localization to the actin filaments. TBL1 mutated in the LisH domain was not imported into the nucleus. Mutations in OFD1 modified its localization to the Golgi apparatus and in some cases also to the nucleus. In summary, the LisH domain may participate in protein dimerization, affect protein half-life, and may influence specific cellular localizations. Our results allow the prediction that mutations withinthe LisH motif are likely to result in pathogenic consequences in genes associated with genetic diseases.     

The structure of the N-terminal domain of the product of the lissencephaly gene Lis1 and its functional implications

Mutations in the Lis1 gene result in lissencephaly (smooth brain), a debilitating developmental syndrome caused by the impaired ability of postmitotic neurons to migrate to their correct destination in the cerebral cortex. Sequence similarities suggest that the LIS1 protein contains a C-terminal seven-blade beta-propeller domain, while the structure of the N-terminal fragment includes the LisH (Lis-homology) motif, a pattern found in over 100 eukaryotic proteins with a hitherto unknown function. We present the 1.75 A resolution crystal structure of the N-terminal domain of mouse LIS1, and we show that the LisH motif is a novel, thermodynamically very stable dimerization domain. The structure explains the molecular basis of a low severity form of lissencephaly.     

Structure of the N-terminal domain of the FOP (FGFR1OP) protein and implications for its dimerization and centrosomal localization

The fibroblast growth factor receptor 1 (FGFR1) oncogene partner, FOP, is a centrosomal protein that is involved in the anchoring of microtubules (MTS) to subcellular structures. The protein was originally discovered as a fusion partner with FGFR1 in oncoproteins that give rise to stem cell myeloproliferative disorders. A subsequent proteomics screen identified FOP as a component of the centrosome. FOP contains a Lis-homology (LisH) motif found in more than 100 eukaryotic proteins. LisH motifs are believed to be involved in microtubule dynamics and organization, cell migration, and chromosome segregation; several of them are associated with genetic diseases. We report here a 1.6A resolution crystal structure of the N-terminal dimerization domain of FOP. The structure comprises an alpha-helical bundle composed of two antiparallel chains, each of them having five alpha-helices. The central part of the dimer contains the LisH domain. We further determined that the FOP LisH domain is part of a longer N-terminal segment that is required, albeit not sufficient, for dimerization and centrosomal localization of FOP.     

The N-terminal coiled-coil of Ndel1 is a regulated scaffold that recruits LIS1 to dynein

Ndel1 has been implicated in a variety of dynein-related processes, but its specific function is unclear. Here we describe an experimental approach to evaluate a role of Ndel1 in dynein-dependent microtubule self-organization using Ran-mediated asters in meiotic Xenopus egg extracts. We demonstrate that extracts depleted of Ndel1 are unable to form asters and that this defect can be rescued by the addition of recombinant N-terminal coiled-coil domain of Ndel1. Ndel1-dependent microtubule self-organization requires an interaction between Ndel1 and dynein, which is mediated by the dimerization fragment of the coiled-coil. Full rescue by the coiled-coil domain requires LIS1 binding, and increasing LIS1 concentration partly rescues aster formation, suggesting that Ndel1 is a recruitment factor for LIS1. The interactions between Ndel1 and its binding partners are positively regulated by phosphorylation of the unstructured C terminus. Together, our results provide important insights into how Ndel1 acts as a regulated scaffold to temporally and spatially regulate dynein.     

Dimerization of CtIP may stabilize in vivo interactions with the Retinoblastoma-pocket domain

CtIP is a tumor suppressor that interacts with Retinoblastoma protein (Rb) to regulate the G1/S-phase transition of the cell cycle. Despite its large size (897 residues) CtIP has few known structured regions. Rather it contains several linear motifs that interact with known binding partners, including an LXCXE motif that binds the pocket domain of Rb-family proteins. This LXCXE motif lies at the C-terminus of the only known structured domain, an N-terminal coiled-coil dimerization domain (DD; residues 45-160). Yeast two-hybrid (Y2H) and GST-pulldown analyses showed that CtIP requires the LXCXE motif to bind the Rb-pocket. Although isothermal titration calorimetry data indicates that the LXCXE motif is the sole determinant of binding affinity for the Rb-pocket domain (K(A) approximately 10(6)M(-1)), Y2H data indicates that the DD is required to stabilize the interaction in vivo. Thus dimerization may increase the apparent stability of the proteins and/or the lifetime of the complexes.     

Stability and specificity of heterodimer formation for the coiled-coil neck regions of the motor proteins Kif3A and Kif3B: the role of unstructured oppositely charged regions.

We investigated the folding, stability, and specificity of dimerization of the neck regions of the kinesin-like proteins Kif3A (residues 356-416) and Kif3B (residues 351-411). We showed that the complementary charged regions found in the hinge regions (which directly follow the neck regions) of these proteins do not adopt any secondary structure in solution. We then explored the ability of the complementary charged regions to specify heterodimer formation for the neck region coiled-coils found in Kif3A and Kif3B. Redox experiments demonstrated that oppositely charged regions specified the formation of a heterodimeric coiled-coil. Denaturation studies with urea demonstrated that the negatively charged region of Kif3A dramatically destabilized its neck coiled-coil (urea1/2 value of 3.9 m compared with 6.7 m for the coiled-coil alone). By comparison, the placement of a positively charged region C-terminal to the neck coiled-coil of Kif3B had little effect on stability (urea1/2 value of 8.2 m compared with 8.8 m for the coiled-coil alone). The pairing of complementary charged regions leads to specific heterodimer formation where the stability of the heterodimeric neck coiled-coil with charged regions had similar stability (urea1/2 value of 7.8 m) to the most stable homodimer (Kif3B) with charged regions (urea1/2 value of 8.0 m) and dramatically more stable than the Kif3A homodimer with charged regions (urea1/2, value of 3.9 m). The heterodimeric coiled-coil with charged extensions has essentially the same stability as the heterodimeric coiled-coil on its own (urea1/2 values of 7.8 and 8.1 m, respectively) suggesting that specificity of heterodimerization is driven by non-specific attraction of the oppositely unstructured charged regions without affecting stability of the heterodimeric coiled-coil.     

Importance of secondary structural specificity determinants in protein folding: insertion of a native beta-sheet sequence into an alpha-helical coiled-coil

To examine how a short secondary structural element derived from a native protein folds when in a different protein environment, we inserted an 11-residue beta-sheet segment (cassette) from human immunoglobulin fold, Fab new, into an alpha-helical coiled-coil host protein (cassette holder). This de novo design protein model, the structural cassette mutagenesis (SCM) model, allows us to study protein folding principles involving both short- and long-range interactions that affect secondary structure stability and conformation. In this study, we address whether the insertion of this beta-sheet cassette into the alpha-helical coiled-coil protein would result in conformational change nucleated by the long-range tertiary stabilization of the coiled-coil, therefore overriding the local propensity of the cassette to form beta-sheet, observed in its native immunoglobulin fold. The results showed that not only did the nucleating helices of the coiled-coil on either end of the cassette fail to nucleate the beta-sheet cassette to fold with an alpha-helical conformation, but also the entire chimeric protein became a random coil. We identified two determinants in this cassette that prevented coiled-coil formation: (1) a tandem dipeptide NN motif at the N-terminal of the beta-sheet cassette, and (2) the hydrophilic Ser residue, which would be buried in the hydrophobic core if the coiled-coil structure were to fold. By amino acid substitution of these helix disruptive residues, that is, either the replacement of the NN motif with high helical propensity Ala residues or the substitution of Ser with Leu to enhance hydrophobicity, we were able to convert the random coil chimeric protein into a fully folded alpha-helical coiled-coil. We hypothesized that this NN motif is a "secondary structural specificity determinant" which is very selective for one type of secondary structure and may prevent neighboring residues from adopting an alternate protein fold. These sequences with secondary structural specificity determinants have very strong local propensity to fold into a specific secondary structure and may affect overall protein folding by acting as a folding initiation site.     

Crystal structure of human GGA1 GAT domain complexed with the GAT-binding domain of Rabaptin5

GGA proteins coordinate the intracellular trafficking of clathrin-coated vesicles through their interaction with several other proteins. The GAT domain of GGA proteins interacts with ARF, ubiquitin, and Rabaptin5. The GGA-Rabaptin5 interaction is believed to function in the fusion of trans-Golgi-derived vesicles to endosomes. We determined the crystal structure of a human GGA1 GAT domain fragment in complex with the Rabaptin5 GAT-binding domain. In this structure, the Rabaptin5 domain is a 90-residue-long helix. At the N-terminal end, it forms a parallel coiled-coil homodimer, which binds one GAT domain of GGA1. In the C-terminal region, it further assembles into a four-helix bundle tetramer. The Rabaptin5-binding motif of the GGA1 GAT domain consists of a three-helix bundle. Thus, the binding between Rabaptin5 and GGA1 GAT domain is based on a helix bundle-helix bundle interaction. The current structural observation is consistent with previously reported mutagenesis data, and its biological relevance is further confirmed by new mutagenesis studies and affinity analysis. The four-helix bundle structure of Rabaptin5 suggests a functional role in tethering organelles.     

Computational modeling suggests dimerization of equine infectious anemia virus Rev is required for RNA binding

Background

The lentiviral Rev protein mediates nuclear export of intron-containing viral RNAs that encode structural proteins or serve as the viral genome. Following translation, HIV-1 Rev localizes to the nucleus and binds its cognate sequence, termed the Rev-responsive element (RRE), in incompletely spliced viral RNA. Rev subsequently multimerizes along the viral RNA and associates with the cellular Crm1 export machinery to translocate the RNA-protein complex to the cytoplasm. Equine infectious anemia virus (EIAV) Rev is functionally homologous to HIV-1 Rev, but shares very little sequence similarity and differs in domain organization. EIAV Rev also contains a bipartite RNA binding domain comprising two short arginine-rich motifs (designated ARM-1 and ARM-2) spaced 79 residues apart in the amino acid sequence. To gain insight into the topology of the bipartite RNA binding domain, a computational approach was used to model the tertiary structure of EIAV Rev.

Results

The tertiary structure of EIAV Rev was modeled using several protein structure prediction and model quality assessment servers. Two types of structures were predicted: an elongated structure with an extended central alpha helix, and a globular structure with a central bundle of helices. Assessment of models on the basis of biophysical properties indicated they were of average quality. In almost all models, ARM-1 and ARM-2 were spatially separated by >15 Å, suggesting that they do not form a single RNA binding interface on the monomer. A highly conserved canonical coiled-coil motif was identified in the central region of EIAV Rev, suggesting that an RNA binding interface could be formed through dimerization of Rev and juxtaposition of ARM-1 and ARM-2. In support of this, purified Rev protein migrated as a dimer in Blue native gels, and mutation of a residue predicted to form a key coiled-coil contact disrupted dimerization and abrogated RNA binding. In contrast, mutation of residues outside the predicted coiled-coil interface had no effect on dimerization or RNA binding.

Conclusions

Our results suggest that EIAV Rev binding to the RRE requires dimerization via a coiled-coil motif to juxtapose two RNA binding motifs, ARM-1 and ARM-2.

     

<Superscript>1</Superscript>H, <Superscript>13</Superscript>C and <Superscript>15</Superscript>N chemical shift assignment of lissencephaly-1 homology (LisH) domain homodimer of human two-hybrid-associated protein 1 with RanBPM (Twa1)

The CTLH complex is a large, highly conserved eukaryotic complex composed of eight proteins that has been associated to several cellular functions, more often described as an E3 ubiquitin ligase complex involved in protein degradation through ubiquitination but also via vacuole-dependent degradation. A common feature observed in several components of this complex is the presence of the domains lissencephaly-1 homology (LisH) and C-terminal to LisH (CTLH). The LisH domain is found in several proteins involved in chromosome segregation, microtubule dynamics, and cell migration. Also, this domain participates in protein dimerization, besides affecting protein half-life, and influencing in specific cellular localization. Among the proteins found in the CTLH complex, Twa1 (Two-hybrid-associated protein 1 with RanBPM), also known as Gid8 (glucose-induced degradation protein 8 homolog) is the smallest, being a good model for structural studies by NMR. In this work we report the chemical shift assignments of the homodimeric LisH domain of Twa1, as a first step to determine its solution structure.     

Exploration of the TRIM Fold of MuRF1 Using EPR Reveals a Canonical Antiparallel Structure and Extended COS-Box

MuRF1 (TRIM63) is a RING-type E3 ubiquitin ligase with a predicted tripartite TRIM fold. TRIM proteins rely upon the correct placement of an N-terminal RING domain, with respect to C-terminal, specific substrate-binding domains. The TRIM domain organization is orchestrated by a central helical domain that forms an antiparallel coiled-coil motif and mediates the dimerization of the fold. MuRF1 has a reduced TRIM composition characterized by a lack of specific substrate binding domains, but contains in its helical domain a conserved sequence motif termed COS-box that has been speculated to fold independently into an α-hairpin. These characteristics had led to question whether MuRF1 adopts a canonical TRIM fold. Using a combination of electron paramagnetic resonance, on spin-labeled protein, and disulfide crosslinking, we show that TRIM63 follows the structural conservation of the TRIM dimerization domain, observed in other proteins. We also show that the COS-box motif folds back onto the dimerization coiled-coil motif, predictably forming a four-helical bundle at the center of the protein and emulating the architecture of canonical TRIMs.     

Crystal structure of a cyanobacterial phytochrome response regulator

The two-component signal transduction pathway widespread in prokaryotes, fungi, molds, and some plants involves an elaborate phosphorelay cascade. Rcp1 is the phosphate receiver module in a two-component system controlling the light response of cyanobacteria Synechocystis sp. via cyanobacterial phytochrome Cph1, which recognizes Rcp1 and transfers its phosphoryl group to an aspartate residue in response to light. Here we describe the crystal structure of Rcp1 refined to a crystallographic R-factor of 18.8% at a resolution of 1.9 A. The structure reveals a tightly associated homodimer with monomers comprised of doubly wound five-stranded parallel beta-sheets forming a single-domain protein homologous with the N-terminal activator domain of other response regulators (e.g., chemotaxis protein CheY). The three-dimensional structure of Rcp1 appears consistent with the conserved activation mechanism of phosphate receiver proteins, although in this case, the C-terminal half of its regulatory domain, which undergoes structural changes upon phosphorylation, contributes to the dimerization interface. The involvement of the residues undergoing phosphorylation-induced conformational changes at the dimeric interface suggests that dimerization of Rcp1 may be regulated by phosphorylation, which could affect the interaction of Rcp1 with downstream target molecules.     

The Cullin-RING E3 ubiquitin ligase CRL4-DCAF1 complex dimerizes via a short helical region in DCAF1

The cullin4A-RING E3 ubiquitin ligase (CRL4) is a multisubunit protein complex, comprising cullin4A (CUL4), RING H2 finger protein (RBX1), and DNA damage-binding protein 1 (DDB1). Proteins that recruit specific targets to CRL4 for ubiquitination (ubiquitylation) bind the DDB1 adaptor protein via WD40 domains. Such CRL4 substrate recognition modules are DDB1- and CUL4-associated factors (DCAFs). Here we show that, for DCAF1, oligomerization of the protein and the CRL4 complex occurs via a short helical region (residues 845-873) N-terminal to DACF1's own WD40 domain. This sequence was previously designated as a LIS1 homology (LisH) motif. The oligomerization helix contains a stretch of four Leu residues, which appear to be essential for α-helical structure and oligomerization. In vitro reconstituted CRL4-DCAF1 complexes (CRL4(DCAF1)) form symmetric dimers as visualized by electron microscopy (EM), and dimeric CRL4(DCAF1) is a better E3 ligase for in vitro ubiquitination of the UNG2 substrate compared to a monomeric complex.     

Charged single alpha-helix: a versatile protein structural motif

A few highly charged natural peptide sequences were recently suggested to form stable alpha-helical structures in water. In this article we show that these sequences represent a novel structural motif called "charged single alpha-helix" (CSAH). To obtain reliable candidate CSAH motifs, we developed two conceptually different computational methods capable of scanning large databases: SCAN4CSAH is based on sequence features characteristic for salt bridge stabilized single alpha-helices, whereas FT_CHARGE applies Fourier transformation to charges along sequences. Using the consensus of the two approaches, a remarkable number of proteins were found to contain putative CSAH domains. Recombinant fragments (50-60 residues) corresponding to selected hits obtained by both methods (myosin 6, Golgi resident protein GCP60, and M4K4 protein kinase) were produced and shown by circular dichroism spectroscopy to adopt largely alpha-helical structure in water. CSAH segments differ substantially both from coiled-coil and intrinsically disordered proteins, despite the fact that current prediction methods recognize them as either or both. Analysis of the proteins containing CSAH motif revealed possible functional roles of the corresponding segments. The suggested main functional features include the formation of relatively rigid spacer/connector segments between functional domains as in caldesmon, extension of the lever arm in myosin motors and mediation of transient interactions by promoting dimerization in a range of proteins.     

Nudf, a fungal homolog of the human LIS1 protein, functions as a dimer in vivo

The NUDF protein is required for nuclear migration through the mycelium of the filamentous fungus Aspergillus nidulans. It is of particular interest, because it closely resembles a human protein, LIS1, that is required for development of the cerebral cortex. Both are approximately 50-kDa proteins with a short N-terminal predicted coiled coil and seven WD-40 domains in the C-terminal half of the molecule. They also interact with homologous proteins, suggesting that they may have similar biochemical functions. Here we describe the purification to homogeneity of NUDF protein in a single step from a cell-free extract of A. nidulans. We demonstrate that NUDF is a homodimer, that its dimerization occurs via the N-terminal coiled coil region of the molecule, and that it must be a dimer to support the growth of A. nidulans.     

Heptad motifs within the distal subdomain of the coiled-coil rod region of M protein from rheumatic fever and nephritis associated serotypes of group A streptococci are distinct from each other: Nucleotide sequence of the M57 gene and relation of the deduced amino acid sequence to other M proteins

Streptococcal M protein, a dimeric alpha helical coiled-coil molecule, is an antigenically variable virulence factor on the surface of the bacteria. Our recent conformational analysis of the complete sequence of the M6 protein led us to propose a basic model for the M protein consisting of an extended central coiled-coil rod domain flanked by a variable N-terminal and a conserved C-terminal end domains. The central coiled-coil rod domain of M protein, which constitutes the major part of the M molecule, is made up of repeating heptads of the generalized sequence a-b-c-d-e-f-g, wherein a and d are predominantly apolar residues. Based on the differences in the heptad pattern of apolar residues and internal sequence homology, the central coiled-coil rod domain of M protein could be further divided into three subdomains I, II, and III. The streptococcal sequelae rheumatic fever (RF) and acute glomerulonephritis (AGN) have been known to be associated with distinct serotypes. Consistent with this, we observed that the AGN associated M49 protein exhibits a heptad motif that is distinct from the RF associated M5 and M6 proteins. Asn and Leu predominated in the a and d positions, respectively, in subdomain I of the M5 and M6 proteins, whereas apolar residues predominated in both these positions in the M49 protein. To establish whether the heptad motif of M49 is unique to this protein, or is a general characteristic of nephritis-associated serotypes, the amino acid sequence of M57, another nephritis-associated serotype, has now been examined. The gene encoding M57 was amplified by PCR, cloned into pUC19 vector, and sequenced. The C-terminal half of M57 is highly homologous to other M proteins (conserved region). In contrast, its N-terminal half (variable region) revealed no significant homology with any of the M proteins. Heptad periodicity analysis of the M57 sequence revealed that the basic design principles, consisting of distinct domains observed in the M6 protein, are also conserved in the M57 molecule. However, the heptad motif within the coiled-coil subdomain I of M57 was distinct from M5 and M6 but similar to M49. Similar analyses of the heptad characteristics within the reported sequences of M1, M12, and M24 proteins further confirmed the conservation of the overall architectural design of sequentially distinct M proteins. Furthermore, the heptad motif within subdomain I of the AGN-associated serotypes M1 and M12 was similar to M49 and M57, whereas that of the RF associated M24 was similar to the M5 and M6 proteins. These results clearly demonstrate a correlation between the heptad motifs within the distal coiled-coil subdomain of the M proteins from different streptococcal serotypes and their epidemiological association with the sequelae AGN and RF.