The structure of the tetratricopeptide repeats of protein phosphatase 5: implications for TPR-mediated protein-protein interactions

Clathrin self-assembly is mediated by a tandemly repeated superhelix

Clathrin is a triskelion-shaped cytoplasmic protein that polymerizes into a polyhedral lattice on intracellular membranes to form protein-coated membrane vesicles. Lattice formation induces the sorting of membrane proteins during endocytosis and organelle biogenesis by interacting with membrane-associated adaptor molecules. The clathrin triskelion is a trimer of heavy-chain subunits (1,675 residues), each binding a single light-chain subunit, in the hub domain (residues 1,074-1,675). Light chains negatively modulate polymerization so that intracellular clathrin assembly is adaptor-dependent. Here we report the atomic structure, to 2.6 A resolution, of hub residues 1,210-1,516 involved in mediating spontaneous clathrin heavy-chain polymerization and light-chain association. The hub fragment folds into an elongated coil of alpha-helices, and alignment analyses reveal a 145-residue motif that is repeated seven times along the filamentous leg and appears in other proteins involved in vacuolar protein sorting. The resulting model provides a three-dimensional framework for understanding clathrin heavy-chain self-assembly, light-chain binding and trimerization.

A role for Ctr9p and Paf1p in the regulation G1 cyclin expression in yeast

Entry into the cell cycle in budding yeast involves transcriptional activation of G1cyclin genes and DNA synthesis genes when cells reach a critical size in late G1. Expression of G1cyclins CLN1 and CLN2 is regulated by the transcription factor SBF (composed of Swi4p and Swi6p) and depends on the cyclin-dependent Cdc28 protein kinase and cyclin Cln3p. To identify novel regulators of SBF-dependent gene expression we screened for mutants that fail to activate transcription of G1cyclins. We found mutations in a gene called CTR9. ctr9 mutants are inviable at 37 degrees C and accumulate large cells. CTR9 is identical to CDP1. CTR9 encodes a conserved nuclear protein of 125 kDa containing several TPR repeats implicated in protein-protein interactions. We show that Ctr9p is a component of a high molecular weight protein complex. Using immuno-affinity chromatography we found that Ctr9p associates with polypeptides of 50 and 65 kDa. By mass spectrometry these were identified as Cdc73p and Paf1p. We show that Paf1p, like Ctr9p, is required for efficient CLN2 transcription, whereas Cdc73p is not. Paf1p and Cdc73p were previously reported to be RNA poly-merase II-associated proteins, suggesting that the Ctr9p complex may interact with the general transcription apparatus.

Turn up the HEAT

The recently determined crystal structure of the PR65/A subunit of protein phosphatase 2A reveals the architecture of proteins containing HEAT repeats. The structural properties of this solenoid protein explain many functional characteristics and account for the involvement of solenoids as scaffold, anchoring and adaptor proteins.

Subunits and substrates of the anaphase-promoting complex

The initiation of anaphase and exit from mitosis depend on a ubiquitination complex called the anaphase-promoting complex (APC) or cyclosome. The APC is composed of more than 10 constitutive subunits and associates with additional regulatory factors in mitosis and during the G1 phase of the cell cycle. At the metaphase-anaphase transition the APC ubiquitinates proteins such as Pds1 in budding yeast and Cut2 in fission yeast whose subsequent degradation by the 26S proteasome is essential for the initiation of sister chromatid separation. Later in anaphase and telophase the APC promotes the inactivation of the mitotic cyclin-dependent protein kinase 1 by ubiquitinating its activating subunit cyclin B. The APC also mediates the ubiquitin-dependent proteolysis of several other mitotic regulators, including other protein kinases, APC activators, spindle-associated proteins, and inhibitors of DNA replication.

Characterization of the Batl (Bacteroides aerotolerance) operon in Bacteroides fragilis: isolation of a B. fragilis mutant with reduced aerotolerance and impaired growth in in vivo model systems

YT135.2.8, a Tn4400' insertion mutant of Bacteroides fragilis strain TM4000, grows poorly when used to infect Monika or Chinese hamster ovary (CHO) cell monolayers and is outcompeted by wild-type strains in mixed infections. YT135.2.8 also shows defects in the rat granuloma pouch model system in monoculture and is completely outcompeted by the wild-type strain in a mixed infection. In addition, this mutant shows defects in a new model system consisting of CHO suspension cell columns. All of these defects may be explained by the finding that YT135.2.8 shows decreased tolerance to exposure to atmospheric oxygen (less aerotolerant). The monolayer growth defect (MGD) of YT135.2.8 can be influenced significantly by the presence of sulphur-containing reducing agents (cysteine, dithiothreitol, thiodiglycol) or the non-sulphur reducing agent Tris-(2-carboxylethyl)phosphine (TCEP). The defects in YT135.2.8 can be complemented by a 6.6 kb fragment of the B. fragilis chromosome. DNA sequencing of this fragment and of the regions flanking the Tn4400' insertion in the B. fragilis chromosome revealed the presence of five open reading frames, corresponding to genes bat (Bacteroides aerotolerance) A, B, C, D, E, which form the Batl operon; Tn4400' inserted within batD. All of the hypothetical proteins possess one or more membrane-spanning domains. BatA and BatB show high similarity to each other but, like BatD, they show no match to sequences of known function in the databases. BatC and BatE contain 2-4 repeated sequences similar to the tetratricopeptide repeats (TPRs) seen in many eukaryotic proteins. The function of TPR sequences in protein interactions in other systems leads to the suggestion that the Bat proteins form a complex. The Batl complex may be involved in the generation or export of reducing power equivalents to the periplasm of the B. fragilis cell.

The identification and characterization of a novel splicing protein, Isy1p, of Saccharomyces cerevisiae

We have identified a novel splicing factor, Isy1p, through two-hybrid screens for interacting proteins involved in nuclear pre-mRNA splicing. Isy1p was tagged and demonstrated to be part of the splicing machinery, associated with spliceosomes throughout the splicing reactions. At least a portion of the Isy1 protein population is associated with snRNAs; low levels of U5 and U6 snRNAs are coimmunoprecipitated specifically with Isy1p. When the ISY1 gene was knocked out, no defect in vegetative growth was observed. Using a sensitive in vivo splicing assay, however, we observed lower splicing efficiency in the isy1 null mutant compared to wild-type, indicating that Isy1 p is important in the optimization of splicing.

The major phosphorylation site of the NADPH oxidase component p67phox is Thr233

Phosphorylation of p67phox was shown to increase two- to three-fold upon stimulation by PMA, N-formylmethionyl-leucylphenylalanine or serum-opsonized zymosan. Phosphopeptide mapping showed one major tryptic peptide for p67phox immunoprecipitated from resting or stimulated cells. In vitro phosphorylation of p67phox by isolated cytosol or mitogen-activated protein kinase also generated the same phosphopeptide. Results of cyanogen bromide digestion and HPLC-MS suggested that Thr233 was the phosphorylated residue. Mutagenesis of Thr233 to alanine resulted in loss of phosphorylation in vitro. In the present work, Thr233 has been identified as the major phosphorylation site of p67phox, which is situated in a proline-rich domain.

smg-7 is required for mRNA surveillance in Caenorhabditis elegans

Eukaryotic mRNAs that contain premature stop codons are degraded more rapidly than their wild-type counterparts, a phenomenon termed "nonsense-mediated mRNA decay" (NMD) or "mRNA surveillance." Functions of six previously described Caenorhabditis elegans genes, smg-1 through smg-6, are required for NMD. Whereas nonsense mutant mRNAs are unstable in smg(+) genetic backgrounds, such mRNAs have normal stability in smg(-) backgrounds. Previous screens for smg mutations have likely not identified all genes involved in NMD, but efforts to identify additional smg genes are limited by the fact that almost 90% of smg mutations identified in genome-wide screens are alleles of smg-1, smg-2, or smg-5. We describe a modified screen for smg mutations that precludes isolating alleles of smg-1, smg-2, and smg-5. Using this screen, we have identified and cloned smg-7, a previously uncharacterized gene that we show is required for NMD. smg-7 is predicted to encode a novel protein that contains an acidic carboxyl terminus and two probable tetratricopeptide repeats. We provide evidence that smg-7 is cotranscribed with the previously characterized gene lin-45 and show that null alleles of smg-7 confer a temperature-sensitive defect in NMD.

Fold prediction and evolutionary analysis of the POZ domain: structural and evolutionary relationship with the potassium channel tetramerization domain

Using iterative database searches, a statistically significant sequence similarity was detected between the POZ (poxvirus and zinc finger) domains found in a variety of proteins involved in animal transcription regulation, cytoskeleton organization, and development, and the tetramerization domain of animal potassium channels. Using the crystal structure of the Aplysia Shaker channel tetramerization domain as a template, the common structure of the POZ domain class was predicted. Examination of the structure resulted in the identification of several structural features and specific amino acid residues that may be involved in conserved protein-protein interactions mediated by the POZ domains as well as those that may contribute to the specificity of these interactions. Phylogenetic analysis of the POZ domains suggests that the common ancestor of the crown group eukaryotes already possessed this domain; POZ domains have undergone independent expansion in plants and in different animal lineages.

Stability and folding of the tumour suppressor protein p16

The tumour suppressor p16 is a member of the INK4 family of inhibi tors of the cyclin D-dependent kinases, CDK4 and CDK6, that are involved in the key growth control pathway of the eukaryotic cell cycle. The 156 amino acid residue protein is composed of four ankyrin repeats (a helix-turn-helix motif) that stack linearly as two four-helix bundles resulting in a non-globular, elongated molecule. The thermodynamic and kinetic properties of the folding of p16 are unusual. The protein has a very low free energy of unfolding, Delta GH-2O/D-N, of 3.1 kcal mol-1 at 25 degreesC. The rate-determining transition state of folding/unfolding is very compact (89% as compact as the native state). The other unusual feature is the very rapid rate of unfolding in the absence of denaturant of 0.8 s-1 at 25 degreesC. Thus, p16 has both thermodynamic and kinetic instability. These features may be essential for the regulatory function of the INK4 proteins and of other ankyrin-repeat-containing proteins that mediate a wide range of protein-protein interactions. The mechanisms of inactivation of p16 by eight cancer-associated mutations were dissected using a systematic method designed to probe the integrity of the secondary structure and the global fold. The structure and folding of p16 appear to be highly vulnerable to single point mutations, probably as a result of the protein's low stability. This vulnerability provides one explanation for the striking frequency of p16 mutations in tumours and in immortalised cell lines.

TTC4, a novel human gene containing the tetratricopeptide repeat and mapping to the region of chromosome 1p31 that is frequently deleted in sporadic breast cancer

The 1p31 region shows loss of heterozygosity in up to 50% of human breast cancers, indicating the presence of a tumor suppressor gene in this location. We have mapped six novel ESTs to a 15-Mb contig of yeast artificial chromosomes spanning the critical region of 1p31. One of these ESTs was localized within the contig to the region most commonly undergoing loss of heterozygosity in breast cancer. The corresponding gene sequence for this EST was established by cDNA cloning and RACE procedures. This gene is 2 kb long and contains a tetratricopeptide repeat motif and a coiled-coil domain. This family of genes has been implicated in a wide variety of functions, including tumorigenesis. This is the fourth member of the human gene family, and so we have named this gene TTC4. Northern blot analysis demonstrates a ubiquitous pattern of gene expression that includes breast tissue. A preliminary screen of human breast cancer cell lines shows that TTC4 is expressed in all cases, but SSCP analysis of the coding region of this gene following RT-PCR failed to reveal any mutations. Clearly, because of its map location, a more extensive analysis is warranted to establish whether subtle mutations are present in breast cancers.

The structure of the protein phosphatase 2A PR65/A subunit reveals the conformation of its 15 tandemly repeated HEAT motifs

The PR65/A subunit of protein phosphatase 2A serves as a scaffolding molecule to coordinate the assembly of the catalytic subunit and a variable regulatory B subunit, generating functionally diverse heterotrimers. Mutations of the beta isoform of PR65 are associated with lung and colon tumors. The crystal structure of the PR65/Aalpha subunit, at 2.3 A resolution, reveals the conformation of its 15 tandemly repeated HEAT sequences, degenerate motifs of approximately 39 amino acids present in a variety of proteins, including huntingtin and importin beta. Individual motifs are composed of a pair of antiparallel alpha helices that assemble in a mainly linear, repetitive fashion to form an elongated molecule characterized by a double layer of alpha helices. Left-handed rotations at three interrepeat interfaces generate a novel left-hand superhelical conformation. The protein interaction interface is formed from the intrarepeat turns that are aligned to form a continuous ridge.

The Myxococcus xanthus pilQ (sglA) gene encodes a secretin homolog required for type IV pilus biogenesis, social motility, and development

The Myxococcus xanthus sglA1 spontaneous mutation was originally isolated because it allowed dispersed cell growth in liquid yet retained the ability to form fruiting bodies. Consequently, most of today's laboratory strains either contain the sglA1 mutation or were derived from strains that carry it. Subsequent work showed that sglA was a gene for social gliding motility, a process which is mediated by type IV pili. Here sglA is shown to map to the major pil cluster and to encode a 901-amino-acid open reading frame (ORF) that is homologous to the secretin superfamily of proteins. Secretins form a channel in the outer membrane for the transport of macromolecules. The closest homologs found were PilQ proteins from Pseudomonas aeruginosa and Neisseria gonorrhoeae, which are required for type IV pili biogenesis and twitching motility. To signify these molecular and functional similarities, we have changed the name of sglA to pilQ. The hypomorphic pilQ1 (sglA1) allele was sequenced and found to contain two missense mutations at residues 741 (G-->S) and 762 (N-->G). In addition, 19 independent social (S)-motility mutations are shown to map to the pilQ locus. In-frame deletions of pilQ and its downstream gene, orfL, were constructed. pilQ is shown to be essential for pilus biogenesis, S-motility, rippling, and fruiting body formation, while orfL is dispensable for these processes. The pilQ1 allele, but not the DeltapilQ allele, was found to render cells hypersensitive to vancomycin, suggesting that PilQ1 alters the permeability properties of the outer membrane. Many differences between pilQ1 and pilQ+ strains have been noted in the literature. We discuss some of these observations and how they may be rationalized in the context of our molecular and functional findings.

The Tup1-Cyc8 protein complex can shift from a transcriptional co-repressor to a transcriptional co-activator

Cyc8(Ssn6)-Tup1, a general co-repressor complex, is recruited to promoter DNA via interactions with DNA-binding regulatory proteins and inhibits the transcription of many different yeast genes. Previous studies have established that repression function of the complex is performed by one subunit of the complex, the Tup1 protein, and requires specific components of the RNA polymerase II holoenzyme such as Sin4 and Rgr1. In this study we test the transcriptional activity of the Cyc8 subunit using a lexA operator-containing reporter. We show that a LexA-Cyc8 hybrid stimulates transcription when expressed in a tup1Delta, a sin4Delta, or a rgr1Delta strain, suggesting that transcriptional activation is an intrinsic property of the Cyc8-Tup1 co-repressor. In support of this notion we demonstrate that Cyc8-Tup1 has a dual function on CIT2, a gene encoding a citrate synthase that is expressed upon mitochondrial dysfunction. First, we show that Cyc8-Tup1 is tethered to CIT2 promoter by interacting with the activation domain of Rtg3, a bHLH/L-Zip DNA-binding transactivator of CIT2. Next we demonstrate that Cyc8-Tup1 activates CIT2 transcription in response to mitochondrial dysfunction, and this stimulatory effect is mediated by Cyc8. In contrast, basal (noninduced) expression of this gene is inhibited by Tup1. These findings establish a positive role for the Cyc8-Tup1 complex in transcription and support a model by which specific metabolic signals may convert the Cyc8-Tup1 transcriptional co-repressor to a co-activator of certain promoters.

Two mutant forms of the S1/TPR-containing protein Rrp5p affect the 18S rRNA synthesis in Saccharomyces cerevisiae

The genetic depletion of yeast Rrp5p results in a synthesis defect of both 18S and 5.8S ribosomal RNAs (Venema J, Tollervey D. 1996. EMBO J 15:5701-5714). We have isolated the RRP5gene in a genetic approach aimed to select for yeast factors interfering with protein import into mitochondria. We describe here a striking feature of Rrp5p amino acid sequence, namely the presence of twelve putative S1 RNA-binding motifs and seven tetratricopeptide repeats (TPR) motifs. We have constructed two conditional temperature-sensitive alleles of RRP5 gene and analyzed them for associated rRNA-processing defects. First, a functional "bipartite gene" was generated revealing that the S1 and TPR parts of the protein can act independently of each other. We also generated a two amino acid deletion in TPR unit 1 (rrp5delta6 allele). The two mutant forms of Rrp5p were shown to cause a defect in 18S rRNA synthesis with no detectable effects on 5.8S rRNA production. However, the rRNA processing pathway was differently affected in each case. Interestingly, the ROK1 gene which, like RRP5, was previously isolated in a screen for synthetic lethal mutations with snR10 deletion, was here identified as a high copy suppressor of the rrp5delta6 temperature-sensitive allele. ROK1 also acts as a low copy suppressor but cannot bypass the cellular requirement for RRP5. Furthermore, we show that suppression by the Rok1p putative RNA helicase rescues the 18S rRNA synthesis defect caused by the rrp5delta6 mutation.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

IFI60/ISG60/IFIT4, a new member of the human IFI54/IFIT2 family of interferon-stimulated genes

We report the cloning and sequencing of a full-length cDNA encoding a new member of the human IFI54 (HGMW-approved symbol IFIT2) gene family, designated IFI60 (HGMW-approved symbol IFIT4). The upstream regulatory region of IFI60 shows conservation in structure with that of the IFI54 and IFI56 (HGMW-approved symbol IFIT1) genes, each containing two interferon-stimulated response elements upstream of a conserved TATA box. We have established a partial gene map of the IFI54 gene family by analysis of YAC library clones. All four members of the human family are clustered together at chromosome 10q23.3. It is proposed that the four members of the IFI54 gene family evolved by a series of duplication events from a common gene of origin.

CNS1 encodes an essential p60/Sti1 homolog in Saccharomyces cerevisiae that suppresses cyclophilin 40 mutations and interacts with Hsp90

Cyclophilins are cis-trans-peptidyl-prolyl isomerases that bind to and are inhibited by the immunosuppressant cyclosporin A (CsA). The toxic effects of CsA are mediated by the 18-kDa cyclophilin A protein. A larger cyclophilin of 40 kDa, cyclophilin 40, is a component of Hsp90-steroid receptor complexes and contains two domains, an amino-terminal prolyl isomerase domain and a carboxy-terminal tetratricopeptide repeat (TPR) domain. There are two cyclophilin 40 homologs in the yeast Saccharomyces cerevisiae, encoded by the CPR6 and CPR7 genes. Yeast strains lacking the Cpr7 enzyme are viable but exhibit a slow-growth phenotype. In addition, we show here that cpr7 mutant strains are hypersensitive to the Hsp90 inhibitor geldanamycin. When overexpressed, the TPR domain of Cpr7 alone complements both cpr7 mutant phenotypes, while overexpression of the cyclophilin domain of Cpr7, full-length Cpr6, or human cyclophilin 40 does not. The open reading frame YBR155w, which has moderate identity to the yeast p60 homolog STI1, was isolated as a high-copy-number suppressor of the cpr7 slow-growth phenotype. We show that this Sti1 homolog Cns1 (cyclophilin seven suppressor) is constitutively expressed, essential, and found in protein complexes with both yeast Hsp90 and Cpr7 but not with Cpr6. Cyclosporin A inhibited Cpr7 interactions with Cns1 but not with Hsp90. In summary, our findings identify a novel component of the Hsp90 chaperone complex that shares function with cyclophilin 40 and provide evidence that there are functional differences between two conserved sets of Hsp90 binding proteins in yeast.

Unc-45 mutations in Caenorhabditis elegans implicate a CRO1/She4p-like domain in myosin assembly

The Caenorhabditis elegans unc-45 locus has been proposed to encode a protein machine for myosin assembly. The UNC-45 protein is predicted to contain an NH2-terminal domain with three tetratricopeptide repeat motifs, a unique central region, and a COOH-terminal domain homologous to CRO1 and She4p. CRO1 and She4p are fungal proteins required for the segregation of other molecules in budding, endocytosis, and septation. Three mutations that lead to temperature-sensitive (ts) alleles have been localized to conserved residues within the CRO1/She4p-like domain, and two lethal alleles were found to result from stop codon mutations in the central region that would prevent translation of the COOH-terminal domain. Electron microscopy shows that thick filament accumulation in vivo is decreased by approximately 50% in the CB286 ts mutant grown at the restrictive temperature. The thick filaments that assemble have abnormal structure. Immunofluorescence and immunoelectron microscopy show that myosins A and B are scrambled, in contrast to their assembly into distinct regions at the permissive temperature and in wild type. This abnormal structure correlates with the high degree of instability of the filaments in vitro as reflected by their extremely low yields and shortened lengths upon isolation. These results implicate the UNC-45 CRO1/She4p-like region in the assembly of myosin isoforms in C. elegans and suggest a possible common mechanism for the function of this UCS (UNC-45/CRO1/She4p) protein family.

Light chain-dependent regulation of Kinesin's interaction with microtubules

We have investigated the mechanism by which conventional kinesin is prevented from binding to microtubules (MTs) when not transporting cargo. Kinesin heavy chain (HC) was expressed in COS cells either alone or with kinesin light chain (LC). Immunofluorescence microscopy and MT cosedimentation experiments demonstrate that the binding of HC to MTs is inhibited by coexpression of LC. Association between the chains involves the LC NH2-terminal domain, including the heptad repeats, and requires a region of HC that includes the conserved region of the stalk domain and the NH2 terminus of the tail domain. Inhibition of MT binding requires in addition the COOH-terminal 64 amino acids of HC. Interaction between the tail and the motor domains of HC is supported by sedimentation experiments that indicate that kinesin is in a folded conformation. A pH shift from 7.2 to 6.8 releases inhibition of kinesin without changing its sedimentation behavior. Endogenous kinesin in COS cells also shows pH-sensitive inhibition of MT binding. Taken together, our results provide evidence that a function of LC is to keep kinesin in an inactive ground state by inducing an interaction between the tail and motor domains of HC; activation for cargo transport may be triggered by a small conformational change that releases the inhibition of the motor domain for MT binding.

Atomic structure of clathrin: a beta propeller terminal domain joins an alpha zigzag linker

Clathrin triskelions form the lattice that organizes recruitment of proteins to coated pits and helps drive vesiculation of the lipid bilayer. We report the crystal structure at 2.6 A resolution of a 55 kDa N-terminal fragment from the 190 kDa clathrin heavy chain. The structure comprises the globular "terminal domain" and the linker that joins it to the end of a triskelion leg. The terminal domain is a seven-blade beta propeller, a structure well adapted to interaction with multiple partners, such as the AP-1 and AP-2 sorting adaptor complexes and the nonvisual arrestins. The linker is an alpha-helical zigzag emanating from the propeller domain. We propose that this simple motif may extend into the rest of the clathrin leg.

Dictyostelium TRFA homologous to yeast Ssn6 is required for normal growth and early development

The TPR (tetratricopeptide repeat) family became widespread during evolution, having been found from bacteria to mammals. By means of restriction enzyme-mediated integration, we have identified a Dictyostelium gene (trfA) highly homologous to a Saccharomyces cerevisiae gene encoding a TPR protein, Ssn6 (Cyc8), which functions as a global transcriptional repressor for diverse genes. The deduced amino acid sequence of the Dictyostelium gene product, TRFA, contains 10 consecutive TPR units as well as Gln repeats, Asn repeats, and a region rich in Glu, Lys, Ser, and Thr. The sequences of some of the 10 TPR units in TRFA are more than 70% identical to the corresponding units in Ssn6. The trfA- cells produced smooth plaques on a bacterial lawn and failed to aggregate normally when starved on a plain agar plate. Individual trfA- cells also failed to correctly respond to cAMP, although the adenylyl cyclase of trfA- cells was expressed upon starvation and activated by stimulation with cAMP as in the wild-type cells. When cultured in a rich medium in suspension, they grew more slowly and stopped growing at a lower density than the wild-type cells. Furthermore, they divided into cells of various sizes and tended to be much smaller than the wild-type cells. These pleiotropic defects of the trfA- cells suggest the possibility that Dictyostelium TRFA may regulate the transcription of diverse genes required for normal growth and early development.

Isolation and characterization of human SGT and identification of homologues in Saccharomyces cerevisiae and Caenorhabditis elegans

We have recently isolated a rat cDNA encoding a novel cellular protein able to interact with the major nonstructural protein NS1 of parvovirus H-1 and have termed this protein SGT, for small glutamine-rich tetratricopeptide repeat (TPR)-containing protein. Here we report the isolation of a cDNA from human placenta encoding the human homologue, human SGT. SGT from rat and human contain 314 and 313 amino acids, respectively, and share 91% sequence identity at the protein level. The highest degree of similarity is present within the central region containing three TPR motifs in tandem array. The similarities, however, also extend beyond this region. Human SGTtranscript was found to be ubiquitously present in all human tissues tested. By fluorescence in situ hybridization analysis we have mapped the human gene to chromosome 19p13. The SGT-coding sequences are evolutionarily conserved, since we could identify genes encoding proteins of similar size and structure in the genomes of Saccharomyces cerevisiae and Caenorhabditis elegans.

Specific binding of tetratricopeptide repeat proteins to the C-terminal 12-kDa domain of hsp90

The molecular chaperone hsp90 in the eukaryotic cytosol interacts with a variety of protein cofactors. Several of these cofactors have protein domains containing tetratricopeptide repeat (TPR) motifs, which mediate binding to hsp90. Using a yeast two-hybrid screen, the 12-kDa C-terminal domain of human hsp90alpha (C90) was found to mediate the interaction of hsp90 with TPR-containing sequences from the hsp90 cofactors FKBP51/54 and FKBP52. In addition, the mitochondrial outer membrane protein hTOM34p was identified as a TPR-containing putative partner protein of hsp90. In experiments with purified proteins, the TPR-containing cofactor p60 (Hop) was shown to form stable complexes with hsp90. A deletion mutant of hsp90 lacking the C90 domain was unable to bind p60, whereas deletion of the approximately 25-kDa N-terminal domain of hsp90 did not affect complex formation. Both p60 and FKBP52 bound specifically to the C90 domain fused to glutathione S-transferase and competed with each other for binding. In reticulocyte lysate, the C90 fusion protein recognized the TPR proteins p60, FKBP52, and Cyp40. Thus, our results identify the C90 domain as the specific binding site for a set of hsp90 cofactors having TPR domains.