Isolation of a mouse cDNA encoding mSTI1, a stress-inducible protein containing the TPR motif

The role of VdSti1 in Verticillium dahliae: insights into pathogenicity and stress responses

Sti1/Hop, a stress-induced co-chaperone protein, serves as a crucial link between Hsp70 and Hsp90 during cellular stress responses. Despite its importance in stress defense mechanisms, the biological role of Sti1 in Verticillium dahliae, a destructive fungal pathogen, remains largely unexplored. This study focused on identifying and characterizing Sti1 homologues in V. dahliae by comparing them to those found in Saccharomyces cerevisiae. The results indicated that the VdSti1-deficient mutant displayed increased sensitivity to drugs targeting the ergosterol synthesis pathway, leading to a notable inhibition of ergosterol biosynthesis. Moreover, the mutant exhibited reduced production of microsclerotia and melanin, accompanied by decreased expression of microsclerotia and melanin-related genes VDH1, Vayg1, and VaflM. Additionally, the mutant's conidia showed more severe damage under heat shock conditions and displayed growth defects under various stressors such as temperature, SDS, and CR stress, as well as increased sensitivity to H2O2, while osmotic stress did not impact its growth. Importantly, the VdSti1-deficient mutant demonstrated significantly diminished pathogenicity compared to the wild-type strain. This study sheds light on the functional conservation and divergence of Sti1 homologues in fungal biology and underscores the critical role of VdSti1 in microsclerotia development, stress response, and pathogenicity of V. dahliae.

Hsp70/Hsp90 Organising Protein (Hop): Coordinating Much More than Chaperones

The Hsp70/Hsp90 organising protein (Hop, also known as stress-inducible protein 1/STI1/STIP1) has received considerable attention for diverse cellular functions in both healthy and diseased states. There is extensive evidence that intracellular Hop is a co-chaperone of the major chaperones Hsp70 and Hsp90, playing an important role in the productive folding of Hsp90 client proteins, although recent evidence suggests that eukaryotic Hop is regulatory within chaperone complexes rather than essential. Consequently, Hop is implicated in many key signalling pathways, including aberrant pathways leading to cancer. Hop is also secreted, and it is now well established that Hop interacts with the prion protein, PrP^C, to mediate multiple signalling events. The intracellular and extracellular forms of Hop most likely represent two different isoforms, although the molecular determinants of these divergent functions are yet to be identified. There is also a growing body of research that reports the involvement of Hop in cellular activities that appear independent of either chaperones or PrP^C. While the various cellular functions of Hop have been described, its biological function remains elusive. However, recent knockout studies in mammals suggest that Hop has an important role in embryonic development. This review provides a critical overview of the latest molecular, cellular and biological research on Hop, critically evaluating its function in healthy systems and how this function is adapted in diseased states.

HOP, a Co-chaperone Involved in Response to Stress in Plants

Protein folding is an essential step for protein functionality. In eukaryotes this process is carried out by multiple chaperones that act in a cooperative manner to maintain the proteome homeostasis. Some of these chaperones are assisted during protein folding by different co-chaperones. One of these co-chaperones is HOP, the HSP70-HSP90 organizing protein. This assistant protein, due to its importance, has been deeply analyzed in other eukaryotes, but its function has only recently started to be envisaged in plants. In this kingdom, the role of HOP has been associated to plant response to different cellular, biotic and abiotic stresses. In this article, we analyze the current knowledge about HOP in eukaryotes, paying a special attention to the recently described roles of HOP in plants. In addition, we discuss the recent breakthroughs in the field and the possible new avenues for the study of plant HOP proteins in the future.

HOP family plays a major role in long-term acquired thermotolerance in Arabidopsis

HSP70-HSP90 organizing protein (HOP) is a family of cytosolic cochaperones whose molecular role in thermotolerance is quite unknown in eukaryotes and unexplored in plants. In this article, we describe that the three members of the AtHOP family display a different induction pattern under heat, being HOP3 highly regulated during the challenge and the attenuation period. Despite HOP3 is the most heat-regulated member, the analysis of the hop1 hop2 hop3 triple mutant demonstrates that the three HOP proteins act redundantly to promote long-term acquired thermotolerance in Arabidopsis. HOPs interact strongly with HSP90 and part of the bulk of HOPs shuttles from the cytoplasm to the nuclei and to cytoplasmic foci during the challenge. RNAseq analyses demonstrate that, although the expression of the Hsf targets is not generally affected, the transcriptional response to heat is drastically altered during the acclimation period in the hop1 hop2 hop3 triple mutant. This mutant also displays an unusual high accumulation of insoluble and ubiquitinated proteins under heat, which highlights the additional role of HOP in protein quality control. These data reveal that HOP family is involved in different aspects of the response to heat, affecting the plant capacity to acclimate to high temperatures for long periods.

HOP3, a member of the HOP family in Arabidopsis, interacts with BiP and plays a major role in the ER stress response

HSP70-HSP90 organizing protein (HOP) is a well-studied family of cytosolic cochaperones. However, the possible role of HOP during the endoplasmic reticulum (ER) stress response and the identity of its interactors within the ER were not previously addressed in any eukaryote. We have demonstrated that Arabidopsis HOP3, whose function was not studied before, interacts in vivo with cytosolic HSP90 and HSP70, and, unexpectedly, with binding immunoglobulin protein (BiP), a HSP70 ER-resident protein. Although BiP lacks the domain described in other eukaryotes for HOP-HSP70 binding, it interacts with HOP3 through a non-canonical association to its nucleotide binding domain. Consistent with this interaction with BiP, HOP3 is partially localized at the ER. Moreover, HOP3 is induced both at transcript and protein levels by unfolded protein response (UPR) inducer agents by a mechanism dependent on inositol-requiring enzyme 1 (IRE1). Importantly, hop3 loss-of-function mutants show a reduction in pollen germination and a hypersensitive phenotype in the presence of ER stress inducer agents, a phenotype that is reverted by the addition of the chemical chaperone tauroursodeoxycholic acid (TUDCA). All these data demonstrate, for the first time in any eukaryote, a main role of HOP as an important regulator of the ER stress response, a process intimately linked in plants to important specific developmental programs and to environmental stress sensing and response.

Quantitative Analysis of Dynamic Protein Interactions during Transcription Reveals a Role for Casein Kinase II in Polymerase-associated Factor (PAF) Complex Phosphorylation and Regulation of Histone H2B Monoubiquitylation

Using affinity purification MS approaches, we have identified a novel role for casein kinase II (CKII) in the modification of the polymerase associated factor complex (PAF-C). Our data indicate that the facilitates chromatin transcription complex (FACT) interacts with CKII and may facilitate PAF complex phosphorylation. Posttranslational modification analysis of affinity-isolated PAF-C shows extensive CKII phosphorylation of all five subunits of PAF-C, although CKII subunits were not detected as interacting partners. Consistent with this, recombinant CKII or FACT-associated CKII isolated from cells can phosphorylate PAF-C in vitro, whereas no intrinsic kinase activity was detected in PAF-C samples. Significantly, PAF-C purifications combined with stable isotope labeling in cells (SILAC) quantitation for PAF-C phosphorylation from wild-type and CKII temperature-sensitive strains (cka1Δ cka2-8) showed that PAF-C phosphorylation at consensus CKII sites is significantly reduced in cka1Δ cka2-8 strains. Consistent with a role of CKII in FACT and PAF-C function, we show that decreased CKII function in vivo results in decreased levels of histone H2B lysine 123 monoubiquitylation, a modification dependent on FACT and PAF-C. Taken together, our results define a coordinated role of CKII and FACT in the regulation of RNA polymerase II transcription through chromatin via phosphorylation of PAF-C.

Stress-Inducible Protein 1 (STI1): Extracellular Vesicle Analysis and Quantification

This chapter is derived from our experience in the study of stress-Inducible Protein 1 (STI1) in extracellular vesicles. We used different techniques to isolate, explore, and characterize the extracellular vesicles that contained this protein. Ultracentrifugation and gel chromatography were used to isolate extracellular vesicles of different sizes, nanotracking particle analysis (NTA) determined number and size of vesicles, while flow cytometry and ELISA were used to determine the specific protein content of vesicles.

Hsp70/Hsp90 organising protein (hop): beyond interactions with chaperones and prion proteins

The Hsp70/Hsp90 organising protein (Hop), also known as stress-inducible protein 1 (STI1), has received considerable attention for diverse cellular functions in both healthy and diseased states. There is extensive evidence that intracellular Hop is a co-chaperone of the major chaperones Hsp70 and Hsp90, playing an important role in the productive folding of Hsp90 client proteins. Consequently, Hop is implicated in a number of key signalling pathways, including aberrant pathways leading to cancer. However, Hop is also secreted and it is now well established that Hop also serves as a receptor for the prion protein, PrP(C). The intracellular and extracellular forms of Hop most likely represent two different isoforms, although the molecular determinants of these divergent functions are yet to be identified. There is also a growing body of research that reports the involvement of Hop in cellular activities that appear independent of either chaperones or PrP(C). While Hop has been shown to have various cellular functions, its biological function remains elusive. However, recent knockout studies in mammals suggest that Hop has an important role in embryonic development. This review provides a critical overview of the latest molecular, cellular and biological research on Hop, critically evaluating its function in healthy systems and how this function is adapted in diseases states.

STI1 antagonizes cytoskeleton collapse mediated by small GTPase Rnd1 and regulates neurite growth

Rnd proteins comprise a branch of the Rho family of small GTP-binding proteins, which have been implicated in rearrangements of the actin cytoskeleton and microtubule dynamics. Particularly in the nervous system, Rnd family proteins regulate neurite formation, dendrite development and axonal branching. A secreted form of the co-chaperone Stress-Inducible Protein 1 (STI1) has been described as a prion protein partner that is involved in several processes of the nervous system, such as neurite outgrowth, neuroprotection, astrocyte development, and the self-renewal of neural progenitor cells. We show that cytoplasmic STI1 directly interacts with the GTPase Rnd1. This interaction is specific for the Rnd1 member of the Rnd family. In the COS collapse assay, overexpression of STI1 prevents Rnd1-plexin-A1-mediated cytoskeleton retraction. In PC-12 cells, overexpression of STI1 enhances neurite outgrowth in cellular processes initially established by Rnd1. Therefore, we propose that STI1 participates in Rnd1-induced signal transduction pathways that are involved in the dynamics of the actin cytoskeleton.

The unconventional secretion of stress-inducible protein 1 by a heterogeneous population of extracellular vesicles

The co-chaperone stress-inducible protein 1 (STI1) is released by astrocytes, and has important neurotrophic properties upon binding to prion protein (PrP(C)). However, STI1 lacks a signal peptide and pharmacological approaches pointed that it does not follow a classical secretion mechanism. Ultracentrifugation, size exclusion chromatography, electron microscopy, vesicle labeling, and particle tracking analysis were used to identify three major types of extracellular vesicles (EVs) released from astrocytes with sizes ranging from 20-50, 100-200, and 300-400 nm. These EVs carry STI1 and present many exosomal markers, even though only a subpopulation had the typical exosomal morphology. The only protein, from those evaluated here, present exclusively in vesicles that have exosomal morphology was PrP(C). STI1 partially co-localized with Rab5 and Rab7 in endosomal compartments, and a dominant-negative for vacuolar protein sorting 4A (VPS4A), required for formation of multivesicular bodies (MVBs), impaired EV and STI1 release. Flow cytometry and PK digestion demonstrated that STI1 localized to the outer leaflet of EVs, and its association with EVs greatly increased STI1 activity upon PrP(C)-dependent neuronal signaling. These results indicate that astrocytes secrete a diverse population of EVs derived from MVBs that contain STI1 and suggest that the interaction between EVs and neuronal surface components enhances STI1-PrP(C) signaling.

Role of stress-inducible protein-1 in recruitment of bone marrow derived cells into the ischemic brains

Stress-inducible protein-1 (STI-1) is the proposed ligand for the cellular prion protein (PrP^C), which is thought to facilitate recovery following stroke. Whether STI-1 expression is affected by stroke and how its signalling facilitates recovery remain elusive. Brain slices from patients that died of ischemic stroke were collected for STI-1 immunohistochemistry. These findings were compared to results from cell cultures, mice with or without the PrP^C knockout, and rats. Based on these findings, molecular and pharmacological interventions were administered to investigate the underlying mechanisms and to test the possibility for therapy in experimental stroke models. STI-1 was upregulated in the ischemic brains from humans and rodents. The increase in STI-1 expression in vivo was not cell-type specific, as it was found in neurons, glia and endothelial cells. Likewise, this increase in STI-1 expression can be mimicked by sublethal hypoxia in primary cortical cultures (PCCs) in vitro, and appear to have resulted from the direct binding of the hypoxia inducible factor-1α (HIF-1α) to the STI-1 promoter. Importantly, this STI-1 signalling promoted bone marrow derived cells (BMDCs) proliferation and migration in vitro and recruitment to the ischemic brain in vivo, and augmenting its signalling facilitated neurological recovery in part by recruiting BMDCs to the ischemic brain. Our results thus identified a novel mechanism by which ischemic insults can trigger a self-protective mechanism to facilitate recovery.

This work identifies HIF-1α-mediated transcription of STI-1 and PrPc interaction as leading to BMDCs recruitment into ischemic brains following stroke in both patients and animal models of stroke, highlighting novel neuroprotective possibilities.

Hsp70 protein complexes as drug targets

Heat shock protein 70 (Hsp70) plays critical roles in proteostasis and is an emerging target for multiple diseases. However, competitive inhibition of the enzymatic activity of Hsp70 has proven challenging and, in some cases, may not be the most productive way to redirect Hsp70 function. Another approach is to inhibit Hsp70's interactions with important co-chaperones, such as J proteins, nucleotide exchange factors (NEFs) and tetratricopeptide repeat (TPR) domain-containing proteins. These co-chaperones normally bind Hsp70 and guide its many diverse cellular activities. Complexes between Hsp70 and co-chaperones have been shown to have specific functions, including roles in pro-folding, pro-degradation and pro-trafficking pathways. Thus, a promising strategy may be to block protein- protein interactions between Hsp70 and its co-chaperones or to target allosteric sites that disrupt these contacts. Such an approach might shift the balance of Hsp70 complexes and re-shape the proteome and it has the potential to restore healthy proteostasis. In this review, we discuss specific challenges and opportunities related to these goals. By pursuing Hsp70 complexes as drug targets, we might not only develop new leads for therapeutic development, but also discover new chemical probes for use in understanding Hsp70 biology.

Knockdown of Hop downregulates RhoC expression, and decreases pseudopodia formation and migration in cancer cell lines

The Hsp90/Hsp70 organising protein (Hop) is a co-chaperone that mediates the interaction of Hsp90 and Hsp70 molecular chaperones during assembly of Hsp90 complexes in cells. Formation of Hsp90 complexes is a key intermediate step in the maturation and homeostasis of oncoproteins and several hormone receptors. In this paper, we demonstrate that knockdown of Hop decreased migration of Hs578T and MDA-MB-231 breast cancer cells. Hop was identified in isolated pseudopodia fractions; it colocalised with actin in lamellipodia, and co-sedimented with purified actin in vitro. Knockdown of Hop caused a decrease in the level of RhoC GTPase, and significantly inhibited pseudopodia formation in Hs578T cells. Our data suggest that Hop regulates directional cell migration by multiple unknown mechanisms.

Thermal stabilization of tissues and the preservation of protein phosphorylation states for two-dimensional gel electrophoresis

2-DE is typically capable of discriminating proteins differing by a single phosphorylation or dephosphorylation event. However, a reliable representation of protein phosphorylation states as they occur in vivo requires that both phosphatases and kinases are rapidly and completely inactivated. Thermal stabilization of mouse cerebral cortex homogenates effectively inactivated these enzymes, as evidenced by comparison with unstabilized tissues where abscissal pI shifts were a common feature in 2-D gels. Of the 588 matched proteins separated on 2-D gels comparing stabilized and unstabilized tissues, 53 proteins exhibited greater than twofold differences in spot volume (ANOVA, p<0.05). Phosphoprotein-specific staining was corroborated by the identification of 16 phosphoproteins by nano-LC MS/MS and phosphotyrosine kinase activity assay.

Identification of Novel Phosphorylation Motifs Through an Integrative Computational and Experimental Analysis of the Human Phosphoproteome

Protein phosphorylation occurs in certain sequence/structural contexts that are still incompletely understood. The amino acids surrounding the phosphorylated residues are important in determining the binding of the kinase to the protein sequence. Upon phosphorylation these sequences also determine the binding of certain domains that specifically bind to phosphorylated sequences. Thus far, such 'motifs' have been identified through alignment of a limited number of well identified kinase substrates. RESULTS: Experimentally determined phosphorylation sites from Human Protein Reference Database were used to identify 1,167 novel serine/threonine or tyrosine phosphorylation motifs using a computational approach. We were able to statistically validate a number of these novel motifs based on their enrichment in known phosphopeptides datasets over phosphoserine/threonine/tyrosine peptides in the human proteome. There were 299 novel serine/threonine or tyrosine phosphorylation motifs that were found to be statistically significant. Several of the novel motifs that we identified computationally have subsequently appeared in large datasets of experimentally determined phosphorylation sites since we initiated our analysis. Using a peptide microarray platform, we have experimentally evaluated the ability of casein kinase I to phosphorylate a subset of the novel motifs discovered in this study. Our results demonstrate that it is feasible to identify novel phosphorylation motifs through large phosphorylation datasets. Our study also establishes peptide microarrays as a novel platform for high throughput kinase assays and for the validation of consensus motifs. Finally, this extended catalog of phosphorylation motifs should assist in a systematic study of phosphorylation networks in signal transduction pathways.

Phosphoproteome dynamics reveal heat-shock protein complexes specific to the Leishmania donovani infectious stage

Leishmania is exposed to a sudden increase in environmental temperature during the infectious cycle that triggers stage differentiation and adapts the parasite phenotype to intracellular survival in the mammalian host. The absence of classical promoter-dependent mechanisms of gene regulation and constitutive expression of most of the heat-shock proteins (HSPs) in these human pathogens raise important unresolved questions as to regulation of the heat-shock response and stage-specific functions of Leishmania HSPs. Here we used a gel-based quantitative approach to assess the Leishmania donovani phosphoproteome and revealed that 38% of the proteins showed significant stage-specific differences, with a strong focus of amastigote-specific phosphoproteins on chaperone function. We identified STI1/HOP-containing chaperone complexes that interact with ribosomal client proteins in an amastigote-specific manner. Genetic analysis of STI1/HOP phosphorylation sites in conditional sti1(-/-) null mutant parasites revealed two phosphoserine residues essential for parasite viability. Phosphorylation of the major Leishmania chaperones at the pathogenic stage suggests that these proteins may be promising drug targets via inhibition of their respective protein kinases.

C. elegans STI-1, the homolog of Sti1/Hop, is involved in aging and stress response

Environmental and physiological stresses such as heat shock, oxidative stress, heavy metals, and pathogenic conditions induce cellular stress response. This response is often mediated by heat shock proteins that function as molecular chaperones. A stress-inducible cochaperone, Sti1/Hop (Hsp organizer protein), functions as an adaptor protein that simultaneously binds with Hsp70 and Hsp90 to transfer client proteins from Hsp70 to Hsp90. However, the biological role of STI-1 in vivo is poorly understood in metazoans. Here, we report the characterization of the Caenorhabditis elegans homolog of Sti1/Hop, which is approximately 56% identical with human STI-1. C. elegans STI-1 (CeSTI-1) is expressed in the pharynx, intestine, nervous system, and muscle from larvae to adults. Analysis of proteins immunoprecipitated with anti-STI-1 antibody by mass spectrometry revealed that CeSTI-1 can bind with both Hsp70 and Hsp90 homologs like its mammalian counterpart. sti-1 expression is elevated by heat stress, and an sti-1(jh125) null mutant shows decreased fertility under heat stress conditions. These mutants also show abnormally high lethality in extreme heat and may be functioning with DAF-16 in thermotolerance. In addition, sti-1(jh125) mutants have a shortened life span. Our results confirm that CeSTI-1 is a cochaperone protein that may maintain homeostatic functions during episodes of stress and can regulate longevity in nematodes.

Sequence analyses reveal that a TPR-DP module, surrounded by recombinable flanking introns, could be at the origin of eukaryotic Hop and Hip TPR-DP domains and prokaryotic GerD proteins

The co-chaperone Hop [heat shock protein (HSP) organising protein] is known to bind both Hsp70 and Hsp90. Hop comprises three repeats of a tetratricopeptide repeat (TPR) domain, each consisting of three TPR motifs. The first and last TPR domains are followed by a domain containing several dipeptide (DP) repeats called the DP domain. These analyses suggest that the hop genes result from successive recombination events of an ancestral TPR-DP module. From a hydrophobic cluster analysis of homologous Hop protein sequences derived from gene families, we can postulate that shifts in the open reading frames are at the origin of the present sequences. Moreover, these shifts can be related to the presence or absence of biological function. We propose to extend the family of Hop co-chaperons into the kingdom of bacteria, as several structurally related genes have been identified by hydrophobic cluster analysis. We also provide evidence of common structural characteristics between hop and hip genes, suggesting a shared precursor of ancestral TPR-DP domains.

Nuclear translocation of the phosphoprotein Hop (Hsp70/Hsp90 organizing protein) occurs under heat shock, and its proposed nuclear localization signal is involved in Hsp90 binding

The Hsp70-Hsp90 complex is implicated in the folding and regulation of numerous signaling proteins, and Hop, the Hsp70-Hsp90 Organizing Protein, facilitates the association of this multichaperone machinery. Phosphatase treatment of mouse cell extracts reduced the number of Hop isoforms compared to untreated extracts, providing the first direct evidence that Hop was phosphorylated in vivo. Furthermore, surface plasmon resonance (SPR) spectroscopy showed that a cdc2 kinase phosphorylation mimic of Hop had reduced affinity for Hsp90 binding. Hop was predominantly cytoplasmic, but translocated to the nucleus in response to heat shock. A putative bipartite nuclear localization signal (NLS) has been identified within the Hsp90-binding domain of Hop. Although substitution of residues within the major arm of this proposed NLS abolished Hop-Hsp90 interaction as determined by SPR, this was not sufficient to prevent the nuclear accumulation of Hop under leptomycin-B treatment and heat shock conditions. These results showed for the first time that the subcellular localization of Hop was stress regulated and that the major arm of the putative NLS was not directly important for nuclear translocation but was critical for Hop-Hsp90 association in vitro. We propose a model in which the association of Hop with Hsp90 and the phosphorylated status of Hop both play a role in the mechanism of nucleo-cytoplasmic shuttling of Hop.

Identification of mouse embryonic stem cell-associated proteins

Over the past few years, there has been a growing interest in discovering the molecular mechanisms controlling embryonic stem cells' (ESCs) proliferation and differentiation. Proteome analysis has proven to be an effective approach to comprehensively unravel the regulatory network of differentiation. We applied a two-dimensional electrophoresis based proteomic approach followed by mass spectrometry to analyze the proteome of two mouse ESC lines, Royan B1 and D3, at 0, 6, and 16 days after differentiation initiation. Out of 97 ESC-associated proteins commonly expressed in two ESC lines, 72 proteins were identified using MALDI TOF-TOF mass spectrometry analysis. The expression pattern of four down-regulated proteins including Hspd1, Hspa8, beta-Actin, and Tpt1 were further confirmed by Western blot and immunofluorescence analyses in Royan B1 and D3 as well as two other mouse ESC lines, Royan C1 and Royan C4. Differential mRNA expression analysis of 20 genes using quantitative real-time reverse transcription PCR revealed a low correlation between mRNA and protein levels during differentiation. We also observed that the mRNA level of Tpt1 increased significantly in differentiating cells, whereas its protein level decreased. Several novel ESC-associated proteins have been presented in this study which warrants further investigation with respect to the etiology of stemness.

Prions: protein only or something more? Overview of potential prion cofactors

Transmissible spongiform encephalopathies (TSEs) in humans and animals are attributed to protein-only infectious agents, called prions. Prions have been proposed to arise from the conformational conversion of the cellular protein PrP(C) into a misfolded form (e.g., PrP(Sc) for scrapie), which precipitates into aggregates and fibrils. It has been proposed that the conversion process is triggered by the interaction of the infectious form (PrP(Sc)) with the cellular form (PrP(C)) or might result from a mutation in the gene for PrP(C). However, until recently, all efforts to reproduce this process in vitro had failed, suggesting that host factors are necessary for prion replication. In this review we discuss recent findings such as the cellular factors that might be involved in the conformational conversion of prion proteins and the potential mechanisms by which they could operate.

Endogenous inhibitors of InsP3-induced Ca2+ release in neuroblastoma cells

Cerebellar Purkinje neurons and neuroblastoma N1E-115 cells require 10-50 times more InsP3 to induce Ca2+ release than do a variety of non-neuronal cells (including astrocytes, hepatocytes, endothelial cells, or smooth muscle cells). Given the importance of InsP3-induced Ca2+ release for the development of synaptic plasticity in Purkinje neurons, a low InsP3 sensitivity may facilitate the integration of numerous synaptic inputs before initiating a change in synaptic strength. In the present study, attention is directed at the mechanism underlying this low InsP3 sensitivity of Ca2+ release. We show that permeabilization of neuroblastoma cells with saponin increased InsP3 sensitivity of Ca2+ release, indicating the presence of a diffusible, cytosolic inhibitor(s) of Ca2+ release. Consistent with this hypothesis, gel filtration of the neuroblastoma cytosol yielded three peaks that inhibited InsP3-induced Ca2+ release from permeabilized cells. The prominent inhibitory peak decreased the InsP3 sensitivity of Ca2+ release from permeabilized cells, did not bind 3H-InsP3, and was present in sufficient levels to account for the low InsP3 sensitivity of Ca2+ release in intact neuroblastoma cells. Purification of this prominent inhibitory fraction yielded a protein band that was identified by mass spectrometry as stress-induced phosphoprotein 1 (mSTI1). Furthermore, immunoprecipitation of mSTI1 decreased the inhibitory activity of N1E-115 cytosol, indicating that mSTI1 contributes to the inhibition of InsP3-induced Ca2+ release. Thus, the low InsP3 sensitivity of Ca2+ release in neuroblastoma cells can be explained by the presence of cytosolic inhibitors of Ca2+ release and include stress-induced phosphoprotein 1.

Expression of stress inducible protein 1 (Stip1) in the mouse testis

Phthalate esters are considered endocrine disruptors that interfere with the endocrine balance and development of the mammalian testis. Mono-2-ethylhexyl phthalate (MEHP), the active metabolite of the ubiquitously used plasticizer di-2-ethylhexyl phthalate (DEHP), acts upon Sertoli cells as initial target. By subtractive cDNA libraries we identified genes deregulated as response to MEHP in primary cultures of mouse Sertoli cells. The expression of mouse stress inducible protein 1 (Stip1) was detected as upregulated as a result of MEHP exposure. Stip1 is a cochaperone protein that is homologous to the human heat shock cognate protein 70 (hsc70)/heat shock protein 90 (hsp90)-organizing protein (Hop). To assess the presence and localization of Stip1 in mouse testis and its potential role in stress defense, we studied the expression pattern of the Stip1 protein by immunohistochemistry and of the mRNA by in situ hybridization. Both the protein and the mRNA of Stip1 were mainly found in the cytoplasm of all types of spermatogonia and spermatocytes up till zygotene, the expression decreased during late pachytene and was very weak in diplotene spermatocytes and round spermatids. Interestingly, this expression pattern resembled the pattern of stress sensitivity of spermatogenic cells in that the most sensitive cell types show the weakest expression of Stip1. This suggests an important role for Stip1 in the ability of germ cells to survive in stress conditions including high temperatures.

PrP cooperates with STI1 to regulate SOD activity in PrP-deficient neuronal cell line

Cellular prion protein (PrP(C)) plays anti-apoptotic and anti-oxidative roles in apoptosis induced by serum deprivation in an immortalized prion protein gene (Prnp)-deficient neuronal cell line. The octapeptide repeat region (OR) and N-terminal half of the hydrophobic region (HR) of PrP(C) are indispensable for PrP(C) activity, but the mechanisms remain unclear. In the present study, elucidation of the mechanisms by which PrP(C) elicits the anti-oxidative activities was facilitated by evidence of stress-inducible protein 1 (STI1) mediating PrP(C)-dependent superoxide dismutase (SOD) activation. Immunoprecipitation revealed that PrP(C) was associated with STI1. The inhibitory peptides against PrP(C)-STI1 binding [STI1 pep.1 and PrP(113-132)] indicated toxic activity in PrP(C)-expressing cells by inhibiting SOD activity but not in Prnp(-/-) cells. Furthermore, OR and N-terminal half of the HR were required for the inhibitory effect of PrP(113-132) but not STI1 pep.1. These data are consistent with results established with a model where OR and N-terminal half of the HR mediate the action of STI1 upon cell survival and upregulation of SOD activity.

Nuclear translocation of the Hsp70/Hsp90 organizing protein mSTI1 is regulated by cell cycle kinases

The co-chaperone murine stress-inducible protein 1 (mSTI1), an Hsp70/Hsp90 organizing protein (Hop) homologue, mediates the assembly of the Hsp70/Hsp90 chaperone heterocomplex. The mSTI1 protein can be phosphorylated in vitro by cell cycle kinases proximal to a putative nuclear localization signal (NLS), which substantiated a predicted casein kinase II (CKII)-cdc2 kinase-NLS (CcN) motif at position 180-239 and suggested that mSTI1 might move between the cytoplasm and the nucleus under certain cell cycle conditions. The mechanism responsible for the cellular localization of mSTI1 was probed using NIH3T3 fibroblasts to investigate the localization of endogenous mSTI1 and enhanced green fluorescent protein (EGFP)-tagged mSTI1 mutants. Localization studies on cell lines stably expressing NLS(mSTI1)-EGFP and EGFP demonstrated that the NLS(mSTI1) was able to promote a nuclear localization of EGFP. The mSTI1 protein was exclusively cytoplasmic in most cells under normal conditions but was present in the nucleus of a subpopulation of cells and accumulated in the nucleus following inhibition of nuclear export (leptomycin B treatment). G1/S-phase arrest (using hydroxyurea) and inhibition of cdc2 kinase (using olomoucine) but not inhibition of casein kinase II (using 5,6-dichlorobenzimidazole riboside), increased the proportion of cells with endogenous mSTI1 nuclear staining. mSTI1-EGFP behaved identically to endogenous mSTI1. The functional importance of key residues was tested using modified mSTI1-EGFP proteins. Inactivation and phosphorylation mimicking of potential phosphorylation sites in mSTI1 altered the nuclear translocation. Mimicking of phosphorylation at the mSTI1 CKII phosphorylation site (S189E) promoted nuclear localization of mSTI1-EGFP. Mimicking phosphorylation at the cdc2 kinase phosphorylation site (T198E) promoted cytoplasmic localization of mSTI1-EGFP at the G1/S-phase transition,whereas removal of this site (T198A) promoted the nuclear localization of mSTI1-EGFP under the same conditions. These data provide the first evidence of nuclear import and export of a major Hsp70/Hsp90 co-chaperone and the regulation of this nuclear-cytoplasmic shuttling by cell cycle status and cell cycle kinases.

Hop: more than an Hsp70/Hsp90 adaptor protein

Molecular chaperones facilitate the correct folding of other proteins under physiological and stress conditions. Recently it has become evident that various co-chaperone proteins regulate the cellular functions of these chaperones, particularly Hsp70 and Hsp90. Hop is one of the most extensively studied co-chaperones that is able to directly associate with both Hsp70 and Hsp90. The current dogma proposes that Hop functions primarily as an adaptor that directs Hsp90 to Hsp70-client protein complexes in the cytoplasm. However, recent evidence suggests that Hop can also modulate the chaperone activities of these Hsps, and that it is not dedicated to Hsp70 and Hsp90. While the co-chaperone function of Hop within the cytoplasm has been extensively studied, its association with nuclear complexes and prion proteins remains to be elucidated. This article will review the structural features of Hop, and the evidence that its biological function is considerably broader than previously envisaged.

AtCHIP, a U-box-containing E3 ubiquitin ligase, plays a critical role in temperature stress tolerance in Arabidopsis

The Arabidopsis gene AtCHIP encodes a protein with three tetratricopeptide repeats and a U-box domain, which is structurally similar to the animal CHIP proteins, a new class of E3 ubiquitin ligases. Like animal CHIP proteins, AtCHIP has E3 ubiquitin ligase activity in vitro. AtCHIP is a single-copy gene, and its transcript is up-regulated by several stress conditions such as low and high temperatures. However, increased AtCHIP expression alone was not correlated with increased stress tolerance; in fact, overexpression of AtCHIP in Arabidopsis rendered plants more sensitive to both low- and high-temperature treatments. Higher electrolyte leakage was observed in leaves of AtCHIP overexpression plants after chilling temperature treatment, suggesting that membrane function is likely impaired in these plants under such a condition. These results indicate that AtCHIP plays an important role in plant cellular metabolism under temperature stress conditions.

Tetratricopeptide repeat motif-mediated Hsc70-mSTI1 interaction. Molecular characterization of the critical contacts for successful binding and specificity

Murine stress-inducible protein 1 (mSTI1) is a co-chaperone that is homologous with the human Hsp70/Hsp90-organizing protein (Hop). Guided by Hop structural data and sequence alignment analyses, we have used site-directed mutagenesis, co-precipitation assays, circular dichroism spectroscopy, steady-state fluorescence, and surface plasmon resonance spectroscopy to both qualitatively and quantitatively characterize the contacts necessary for the N-terminal tetratricopeptide repeat domain (TPR1) of mSTI1 to bind to heat shock cognate protein 70 (Hsc70) and to discriminate between Hsc70 and Hsp90. We have shown that substitutions in the first TPR motif of Lys(8) or Asn(12) did not affect binding of mSTI1 to Hsc70, whereas double substitution of these residues abrogated binding. A substitution in the second TPR motif of Asn(43) lowered but did not abrogate binding. Similarly, a deletion in the second TPR motif coupled with a substitution of Lys(8) or Asn(12) reduced but did not abrogate binding. These results suggest that mSTI1-Hsc70 interaction requires a network of interactions not only between charged residues in the TPR1 domain of mSTI1 and the EEVD motif of Hsc70 but also outside the TPR domain. We propose that the electrostatic interactions in the first TPR motif made by Lys(8) or Asn(12) define part of the minimum interactions required for successful mSTI1-Hsc70 interaction. Using a truncated derivative of mSTI1 incapable of binding to Hsp90, we substituted residues on TPR1 potentially involved in hydrophobic contacts with Hsc70. The modified protein had reduced binding to Hsc70 but now showed significant binding capacity for Hsp90. In contrast, topologically equivalent substitutions on a truncated derivative of mSTI1 incapable of binding to Hsc70 did not confer Hsc70 specificity on TPR2A. Our results suggest that binding of Hsc70 to TPR1 is more specific than binding of Hsp90 to TPR2A with serious implications for the mechanisms of mSTI1 interactions with Hsc70 and Hsp90 in vivo.

Characterization of a plant homolog of hop, a cochaperone of hsp90

The 90-kD molecular chaperone hsp90 is the key component of a multiprotein chaperone complex that facilitates folding, stabilization, and functional modulation of a number of signaling proteins. The components of the animal chaperone complex include hsp90, hsp70, hsp40, Hop, and p23. The animal Hop functions to link hsp90 and hsp70, and it can also inhibit the ATPase activity of hsp90. We have demonstrated the presence of an hsp90 chaperone complex in plant cells, but not all components of the complex have been identified. Here, we report the isolation and characterization of soybean (Glycine max) GmHop-1, a soybean homolog of mammalian Hop. An analysis of soybean expressed sequence tags, combined with preexisting data in literature, suggested the presence of at least three related genes encoding Hop-like proteins in soybean. Transcripts corresponding to Hop-like proteins in soybean were detected under normal growth conditions, and their levels increased further in response to stress. A recombinant GmHop-1 bound hsp90 and its binding to hsp90 could be blocked by the tetratricopeptide repeat (TPR) domain of rat (Rattus norvegicus) protein phosphatase 5. Deletion of amino acids 325 to 395, adjacent to the TPR2A domain in GmHop-1, resulted in loss of hsp90 binding. In a minimal assembly system, GmHop-1 was able to stimulate mammalian steroid receptor folding. These data show that plant and animal Hop homologs are conserved in their general characteristics, and suggest that a Hop-like protein in plants is an important cochaperone of plant hsp90.

Stress-inducible protein 1 is a cell surface ligand for cellular prion that triggers neuroprotection

Prions are composed of an isoform of a normal sialoglycoprotein called PrP(c), whose physiological role has been under investigation, with focus on the screening for ligands. Our group described a membrane 66 kDa PrP(c)-binding protein with the aid of antibodies against a peptide deduced by complementary hydropathy. Using these antibodies in western blots from two-dimensional protein gels followed by sequencing the specific spot, we have now identified the molecule as stress-inducible protein 1 (STI1). We show that this protein is also found at the cell membrane besides the cytoplasm. Both proteins interact in a specific and high affinity manner with a K(d) of 10(-7) M. The interaction sites were mapped to amino acids 113-128 from PrP(c) and 230-245 from STI1. Cell surface binding and pull-down experiments showed that recombinant PrP(c) binds to cellular STI1, and co-immunoprecipitation assays strongly suggest that both proteins are associated in vivo. Moreover, PrP(c) interaction with either STI1 or with the peptide we found that represents the binding domain in STI1 induce neuroprotective signals that rescue cells from apoptosis.

Gene expression profiling following BMP-2 induction of mesenchymal chondrogenesis in vitro

OBJECTIVE

This study aims to apply gene expression profiling technology to gain insight into the molecular regulation of mesenchymal chondrogenesis.

METHODS

The experimental system consists of micromass cultures of C3H10T1/2 cells, a murine multipotential embryonic cell line, treated with the chondroinductive growth factor, bone morphogenetic factor-2 (BMP-2). In this system, chondrogenic differentiation characterized by both morphological changes and cartilage matrix gene expression has been shown to be completely dependent upon BMP-2 treatment and the high cell plating density of micromass cultures. To identify candidate genes that may have key functional roles in chondrogenesis, we have applied subtractive hybridization to isolate genes whose expression is significantly up- or down-regulated during chondrogenesis. RNA was isolated from micromass cultures treated with BMP-2 for 24 h and analysed for representational differences by means of a subtractive hybridization screening method.

RESULTS

Sixteen different genes were identified whose expression was up-regulated between two- and 12-fold by B,P-2, and twelve different genes were identified whose expression was down-regulated between two- and seven-fold by BMP-2.

CONCLUSIONS

The potential of this screening methodology to identify new BMP-2 regulated genes is suggested by the fact that a majority of the identified genes are indeed novel. Identification and characterization of these genes should provide insight as to how chondrogenesis is regulated and also should provide important new markers for the study of osteoarthritis.

The cochaperone murine stress-inducible protein 1: overexpression, purification, and characterization

Murine stress-inducible protein 1 (mSTI1) is a cochaperone that is homologous with the human heat shock cognate protein 70 (Hsc70)/heat shock protein 90 (Hsp90)-organizing protein (Hop). To analyze the biochemical properties of mSTI1 and the stoichiometry of the Hsc70.mSTI1.Hsp90 association, recombinant mSTI1 was produced in untagged, histidine (His)-tagged, and glutathione S-transferase (GST)-tagged forms. His-mSTI1 was detected either as a dimer during size-exclusion-high-performance liquid chromatography (SE-HPLC) or as a monomer during Superdex 200 gel filtration chromatography. SE-HPLC on GST-mSTI1 and untagged mSTI1 suggested that mSTI1 existed as a monomer. Cross-linking of His-mSTI1 detected a compact monomeric species and a dimeric species. Gel filtration on the association of bovine STI1 or His-mSTI1 with Hsc70 detected species of molecular mass consistent with a dimeric STI1 species or a 1:1 complex of STI1 and Hsc70. Our data and that of others suggest that mSTI1 and its homologues exist as either a monomer or a dimer and that this facilitates its proposed function as an Hsc70/Hsp90 organizing protein.

Transcriptome analysis of channel catfish (Ictalurus punctatus): genes and expression profile from the brain

Expressed sequence tag (EST) analysis was conducted using a complementary DNA (cDNA) library made from the brain mRNA of channel catfish (Ictalurus punctatus). As part of our transcriptome analysis in catfish to develop molecular reagents for comparative functional genomics, here we report analysis of 1201 brain cDNA clones. Of the 1201 clones, 595 clones (49.5%) were identified as known genes by BLAST searches and 606 clones (50.5%) as unknown genes. The 595 clones of known gene products represent transcripts of 251 genes. These known genes were categorized into 15 groups according to their biological functions. The largest group of known genes was the genes involved in translational machinery (21.4%) followed by mitochondrial genes (6.2%), structural genes (3.1%), genes homologous to sequences of unknown functions (2.3%), enzymes (2.7%), hormone and regulatory proteins (2.5%), genes involved in immune systems (2.1%), genes involved in sorting, transport, and metal metabolism (1.8%), transcriptional factors and DNA repair proteins (1.6%), proto-oncogenes (1.2%), lipid binding proteins (1.2%), stress-induced genes (0.7%), genes homologous to human genes involved in mental diseases (0.6%), and development or differentiation-related genes (0.3%). The number of genes represented by the 606 clones of unknown genes is not known at present, but the high percentage of clones showing no homology to any known genes in the GenBank databases may indicate that a great number of novel genes exist in teleost brain.

The in vitro phosphorylation of the co-chaperone mSTI1 by cell cycle kinases substantiates a predicted casein kinase II-p34cdc2-NLS (CcN) motif

The co-chaperone murine stress-inducible protein 1 (mSTI1), a Hsp70/Hsp90 organizing protein (Hop) homolog, functions as a physical link between Hsp70 and Hsp90 by mediating the formation of the mSTI1/ Hsp70/Hsp90 chaperone heterocomplex. We show here that mSTI1 is an in vitro substrate of cell cycle kinases. Casein kinase II (CKII) phosphorylates mSTI1 at S189, and cdc2 kinase (p34cdc2) at T198, substantiating a predicted CKII-p34cdc2-NLS (CcN) motif. The possible implications of this phosphorylation as a cell cycle checkpoint are discussed.

Heat shock cognate protein 70 chaperone-binding site in the co-chaperone murine stress-inducible protein 1 maps to within three consecutive tetratricopeptide repeat motifs

Murine stress-inducible protein 1 (mSTI1) is a co-chaperone homologous with the human heat shock cognate protein 70 (hsc70)/heat shock protein 90 (hsp90)-organizing protein (Hop). The concomitant interaction of mSTI1 with hsp70 and hsp90 at its N- and C-termini respectively is mediated by the tetratricopeptide repeat (TPR) motifs in these regions. With the use of co-precipitation assays, we show here that the N-terminal TPR domain of mSTI1 without extensive flanking regions is both necessary and sufficient to mediate a specific interaction with hsc70. In contrast, other TPR-containing co-chaperones require TPR flanking regions for target substrate recognition, suggesting different mechanisms of TPR-mediated chaperone-co-chaperone interactions. Furthermore, the interaction between mSTI1 and hsc70 was analysed to ascertain the effect of replacing or deleting conserved amino acid residues and sequences within the three TPR motifs constituting the N-terminal TPR domain of full-length mSTI1. Replacement of a bulky hydrophobic residue in TPR1 disrupted the interaction of mSTI1 with hsc70. A highly conserved sequence in TPR2 was altered by deletion or single amino acid replacement. These derivatives retained a specific interaction with hsc70. These results are consistent with a model in which conserved residues within the N-terminal TPR region of mSTI1 contribute differentially to the interaction with hsc70, and in which TPR1 has a significant role in targeting mSTI1 to hsc70. The contribution of the TPR domain mutations and deletions are discussed with respect to their effect on target substrate interactions.

The tetratricopeptide repeat: a structural motif mediating protein-protein interactions

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel alpha-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and there is extensive evidence indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932-939.

The tetratricopeptide repeat: a structural motif mediating protein-protein interactions

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel alpha-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and there is extensive evidence indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932-939.

Regulation of imaginal disc cell size, cell number and organ size by Drosophila class I(A) phosphoinositide 3-kinase and its adaptor

BACKGROUND

Class I(A) phosphoinositide 3-kinases (PI 3-kinases) have been implicated in the regulation of several cellular processes including cell division, cell survival and protein synthesis. The size of Drosophila imaginal discs (epithelial structures that give rise to adult organs) is maintained by factors that can compensate for experimentally induced changes in these PI 3-kinase-regulated processes. Overexpression of the gene encoding the Drosophila class I(A) PI 3-kinase, Dp110, in imaginal discs, however, results in enlarged adult organs. These observations have led us to investigate the role of Dp100 and its adaptor, p60, in the control of imaginal disc cell size, cell number and organ size.

RESULTS

Null mutations in Dp110 and p60 were generated and used to demonstrate that they are essential genes that are autonomously required for imaginal disc cells to achieve their normal adult size. In addition, modulating Dp110 activity increases or reduces cell size in the developing imaginal disc, and does so throughout the cell cycle. The inhibition of Dp110 activity reduces the rate of increase in cell number in the imaginal discs, suggesting that Dp110 normally promotes cell division and/or cell survival. Unlike direct manipulation of cell-cycle progression, manipulation of Dp110 activity in one compartment of the disc influences the size of that compartment and the size of the disc as a whole.

CONCLUSIONS

We conclude that during imaginal disc development, Dp110 and p60 regulate cell size, cell number and organ size. Our results indicate that Dp110 and p60 signalling can affect growth in multiple ways, which has important implications for the function of signalling through class I(A) PI 3-kinases.

Bacterial lipopolysaccharide induces expression of the stress response genes hop and H411

CD14-transfected Chinese hamster ovary K1 fibroblasts (CHO/CD14) respond to lipopolysaccharide (LPS) by metabolizing arachidonic acid and with translocation of NF-kappaB to the nucleus. Although previous experiments failed to identify the production of tumor necrosis factor-alpha and interleukin (IL)-1beta by CHO/CD14 cells, LPS did induce the expression of IL-6 mRNA and the subsequent release of the IL-6 protein. To identify additional LPS-inducible genes, a cDNA library derived from LPS-stimulated CHO/CD14 cells was screened by subtractive hybridization. Fourteen genes were found to be expressed differentially, and two were analyzed in detail: hop (Hsp70/Hsp90-organizing protein), which is the hamster homologue of the stress-inducible yeast gene, STI1, and clone H411, which encodes a novel LPS-inducible growth factor. In response to LPS, the expression of Hop mRNA was also increased in both the murine macrophage cell line, RAW 264.7, as well as in primary hamster macrophages. This suggested that the up-regulation of Hop expression is part of the macrophage stress response to LPS. Clone H411 encodes a protein in the epidermal growth factor-like repeat protein family. Overexpression of H411 cDNA in the RAW 264.7 macrophage cell line promoted an increased growth rate, suggesting that expression of H411 is part of the proliferative cell response to LPS. Both Hop and H411 represent novel gene products not previously recognized as part of the complex biological response to endotoxin.

Developmentally regulated expression of mtprd, the murine ortholog of tprd, a gene from the Down syndrome chromosomal region 1

The gene tprd, which contains three tetratricopeptide domains, has been recently localized in the Down syndrome (DS) chromosomal region 1. We have cloned a cDNA encoding part of the murine ortholog of tprd and used it to characterize the expression pattern of this gene during development and at the adult stage. At E8.5 the expression is uniform. In the later stages of embryogenesis, although expression remains ubiquitous, a pattern of tissues with particularly high expression develops: the strong expression is restricted to non proliferating zones of the nervous system such as the external layer of the cortex, the spinal cord, the cranial and root ganglia and the nerves. In the brain of adult mouse the strongest signals are observed in layers II-III and V-VI of the cortex, in the hippocampus and in the cerebellum, which correspond to the abnormal brain regions seen in DS patients.

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.