Continuous formation of small clusters with LGR5-positive cells contributes to tumor growth in a colorectal cancer xenograft model.

New cancer characteristics can be discovered by focusing on the process of tumor formation. Cancer stem cells (CSCs) are a key subpopulation, as they are theorized to be at the apex of the tumor hierarchy. We can better understand their function in the tumor hierarchy by using sectioned samples to observe the growth of tumors from their origins as CSCs. In this study, we evaluated the growth of moderate differentiated colorectal cancer from LGR5-positive cells, which is a CSC marker of colorectal cancer, using xenograft and three-dimensional culture models spatiotemporally. These cells express LGR5 at high levels and show CSC phenotypes. To detect them, we used a previously generated antibody that specifically targets LGR5, and were therefore able to observe LGR5-positive cells aggregating into small clusters (sCLs) over the course of tumor growth. Because these LGR5-expressing sCLs formed continuously during growth mainly in the invasive front, we concluded that the structure must contribute significantly to the expansion of CSCs and to tumor growth overall. We confirmed the formation of sCLs from gland structures using a three-dimensional culture model. In addition, sCLs exhibited upregulated genes related to stress response and partial/hybrid epithelial-mesenchymal transition (EMT), as well as genes reported to be prognosis factors. Finally, sCLs with high LGR5 expression were identified in clinical samples. Based on these results, we elucidate how sCLs are an important contributors to tumor growth and the expansion of CSCs.

Tissue-specific effects of an anti-desmoglein-3 ADCC antibody due to expression of the target antigen and physiological characteristics of the mouse vagina.

Desmoglein-3 (DSG3) is a potential target of cytotoxic antibody therapy for squamous cell carcinomas but is also expressed in various normal squamous epithelia. We obtained information about DSG3 distribution in mouse tissues by immunohistochemistry and conducted an intravenous multiple-dose study in mouse to estimate the toxic potential of anti-DSG3 therapy. DSG3 was expressed in the squamous epithelium of several organs including the skin, esophagus, tongue, forestomach, eye, and vagina. It was expressed at all estrous cycles of the vagina with changes in distribution patterns along with the structural changes in each cycle, and expression was reduced in ovariectomized (OVX) mice. On the administration of the antibody, there was disarrangement of the vaginal mucosal epithelium with formation of miroabscess, increased granulocyte infiltration, and single cell necrosis. Despite similar expression levels of DSG3 in other tissues, histopathological changes were limited to the vagina. The severity of the changes was reduced by ovariectomy. From these findings, the lesions were thought to be related to the drastic change in the histological structure of the vaginal mucosa accompanying the estrous cycle. Thus, we have shown that the changing expression of target antigen distribution and its relationship with physiological changes in tissue structure are important features for estimating the toxic potential of cytotoxic antibody therapy.

Difference in morphology and interactome profiles between orthotopic and subcutaneous gastric cancer xenograft models.

In xenograft models, orthotopic (ORT) engraftment is thought to provide a different tumor microenvironment compared with subcutaneous (SC) engraftment. We attempted to characterize the biological difference between OE19 (adenocarcinoma of the gastroesophageal junction) SC and ORT models by pathological analysis and CASTIN (CAncer-STromal INteractome) analysis, which is a novel method developed to analyze the tumor-stroma interactome framework. In SC models, SCID mice were inoculated subcutaneously with OE19 cells, and tumor tissues were sampled at 3 weeks. In ORT models, SCID mice were inoculated under the serosal membrane of the stomach wall, and tumor tissues were sampled at 3 and 6 weeks after engraftment. Results from the two models were then compared. Histopathologically, the SC tumors were well circumscribed from the adjacent tissue, with scant stroma and the formation of large ductal structures. In contrast, the ORT tumors were less circumscribed, with small ductal structures invading into abundant stroma. Then we compared the transcriptome profiles of human tumor cells with the mouse stromal cells of each model by species-specific RNA sequencing. With CASTIN analysis, we successfully identified several interactions that are known to affect the tumor microenvironment as being selectively enhanced in the ORT model. In conclusion, pathological analysis and CASTIN analysis revealed that ORT models of OE19 cells have a more invasive character and enhanced interaction with stromal cells compared with SC models.

Development of rat duodenal monolayer model with effective barrier function from rat organoids for ADME assay.

The in-depth analysis of the ADME profiles of drug candidates using in vitro models is essential for drug development since a drug's exposure in humans depends on its ADME properties. In contrast to efforts in developing human in vitro absorption models, only a limited number of studies have explored models using rats, the most frequently used species in in vivo DMPK studies. In this study, we developed a monolayer model with an effective barrier function for ADME assays using rat duodenal organoids as a cell source. At first, we developed rat duodenal organoids according to a previous report, but they were not able to generate a confluent monolayer. Therefore, we modified organoid culture protocols and developed cyst-enriched organoids; these strongly promoted the formation of a confluent monolayer. Furthermore, adding valproic acid to the culture accelerated the differentiation of the monolayer, which possessed an effective barrier function and apicobasal cell polarity. Drug transporter P-gp function as well as CYP3A activity and nuclear receptor function were confirmed in the model. We expect our novel monolayer model to be a useful tool for elucidating drug absorption processes in detail, enabling the development of highly absorbable drugs.

A long-acting anti-IL-8 antibody improves inflammation and fibrosis in endometriosis.

Current pharmacological treatments for endometriosis are limited to hormonal agents that can relieve pain but cannot cure the disease. Therefore, the development of a disease-modifying drug for endometriosis is an unmet medical need. By studying human endometriotic samples, we found that the progression of endometriosis was associated with the development of inflammation and fibrosis. In addition, IL-8 expression was highly up-regulated in endometriotic tissues and closely correlated with disease progression. We created a long-acting recycling antibody against IL-8 (AMY109) and evaluated its clinical potency. Because rodents do not produce IL-8 and do not experience menstruation, we analyzed the lesions in cynomolgus monkeys that spontaneously developed endometriosis and in a surgically induced endometriosis monkey model. Both spontaneously developed and surgically induced endometriotic lesions demonstrated pathophysiology that was highly similar to that of human endometriosis. Once-a-month subcutaneous injection of AMY109 to monkeys with surgically induced endometriosis reduced the volume of nodular lesions, lowered the Revised American Society for Reproductive Medicine score as modified for monkeys, and ameliorated fibrosis and adhesions. In addition, experiments using cells derived from human endometriosis revealed that AMY109 inhibited the recruitment of neutrophils to endometriotic lesions and the production of monocyte chemoattractant protein-1 from neutrophils. Thus, AMY109 may represent a disease-modifying therapy for patients with endometriosis.

Generation of a lung squamous cell carcinoma three-dimensional culture model with keratinizing structures.

Tumor nests in lung squamous cell carcinoma (LUSC) have a hierarchical structure resembling squamous epithelium. The nests consist of basal-like cells on the periphery and layers of keratinocyte-like cells that differentiate towards the center of the nest, forming keratin pearls. Reproducing this spatial heterogeneity in in vitro models would be useful for understanding the biology of LUSC. Here, we established a three-dimensional (3D) culture model with a squamous epithelial structure using LUSC cell lines PLR327F-LD41 and MCC001F, established in-house. When PLR327F-LD41 cells were cultured in a mixture of Matrigel and collagen I, they generated 3D colonies (designated cancer organoids, or COs) with involucrin (IVL)-positive keratinizing cells in the center (IVLinner COs). COs with uniform size were generated by seeding PLR327F-LD41 cells in a form of small cell aggregates. Since Notch signaling induces the differentiation of squamous epithelium, we confirmed the effect of γ-secretase inhibitor in inhibiting Notch signaling in IVLinner COs. Surprisingly, γ-secretase inhibitor did not block induction of IVL-positive cells; however, cells residing between the CK5-positive basal-like layer and IVL-positive layer decreased significantly. Thus, our 3D culture model with uniform size and structure promises to be a useful tool for elucidating the biology of LUSC and for screening drug-candidates.

Generation of a humanized anti-glypican 3 antibody by CDR grafting and stability optimization.

Glypican 3 (GPC3), a glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan, is expressed in a majority of hepatocellular carcinoma tissues. The murine monoclonal antibody GC33 that specifically binds to the COOH-terminal part of GPC3 causes strong antibody-dependent cellular cytotoxicity against hepatocellular carcinoma cells and exhibits strong antitumor activity in the xenograft models. To apply GC33 for clinical use, we generated a humanized GC33 from complementarity-determining region grafting with the aid of both the hybrid variable region and two-step design methods. The humanized antibody bound to GPC3 specifically and induced antibody-dependent cellular cytotoxicity as effectively as a chimeric GC33 antibody. To improve stability of the humanized GC33, we further optimized humanized GC33 by replacing the amino acid residues that may affect the structure of the variable region of a heavy chain. Substitution of Glu6 with Gln in the heavy chain significantly improved the stability under high temperatures. GC33 also has the risk of deamidation of the -Asn-Gly- sequence in the complementarity-determining region 1 of the light chain. As substitution of Asn diminished the antigen binding, we changed the neighboring Gly to Arg to avoid deamidation. The resulting humanized anti-GPC3 antibody was as efficacious as chimeric GC33 against the HepG2 xenograft and is now being evaluated in clinical trials.

Three-dimensional culture models mimic colon cancer heterogeneity induced by different microenvironments.

Colorectal cancer demonstrates intra-tumour heterogeneity formed by a hierarchical structure comprised of cancer stem cells (CSCs) and their differentiated progenies. The mechanism by which CSCs are maintained and differentiated needs to be further elucidated, and there is evidence that the tumour microenvironment governs cancer stemness. Using PLR123, a colon cancer cell line with CSC properties, we determined the culture conditions necessary to establish a pair of three-dimensional (3D) culture models grown in Matrigel, designated stemCO and diffCO. The conditions were determined by comparing the phenotypes in the models with PLR123 mouse xenografts colonising lung and liver. StemCO resembled LGR5-positive undifferentiated tumours in the lung, and diffCO had lumen structures composed of polarised cells that were similar to the ductal structures found in differentiated tumours in the liver. In a case using the models for biomedical research, treatment with JAG-1 peptide or a γ-secretase inhibitor modified the Notch signaling and induced changes indicating that the signal participates in lumen formation in the models. Our results demonstrate that culture conditions affect the stemness of 3D culture models generated from CSCs and show that comparing models with different phenotypes is useful for studying how the tumour environment regulates cancer.

Visualization of different carrier concentrations in n-type-GaN semiconductors by phase-shifting electron holography with multiple electron biprisms.

Phase-shifting electron holography (PS-EH) using a transmission electron microscope (TEM) was applied to visualize layers with different concentrations of carriers activated by Si (at dopant levels of 1019, 1018, 1017 and 1016 atoms cm-3) in n-type GaN semiconductors. To precisely measure the reconstructed phase profiles in the GaN sample, three electron biprisms were used to obtain a series of high-contrast holograms without Fresnel fringes generated by a biprism filament, and a cryo-focused-ion-beam (cryo-FIB) was used to prepare a uniform TEM sample with less distortion in the wide field of view. All layers in a 350-nm-thick TEM sample were distinguished with 1.8-nm spatial resolution and 0.02-rad phase-resolution, and variations of step width in the phase profile (corresponding to depletion width) at the interfaces between the layers were also measured. Thicknesses of the active and inactive layers at each dopant level were estimated from the observed phase profile and the simulation of theoretical band structure. Ratio of active-layer thickness to total thickness of the TEM sample significantly decreased as dopant concentration decreased; thus, a thicker TEM sample is necessary to visualize lower carrier concentrations; for example, to distinguish layers with dopant concentrations of 1016 and 1015 atoms cm-3. It was estimated that sample thickness must be more than 700 nm to make it be possible to detect sub-layers by the combination of PS-EH and cryo-FIB.

Anti-glypican 3 antibodies cause ADCC against human hepatocellular carcinoma cells.

Glypican 3 (GPC3), a GPI-anchored heparan sulfate proteoglycan, is expressed in the majority of hepatocellular carcinoma (HCC) tissues. Using MRL/lpr mice, we successfully generated a series of anti-GPC3 monoclonal antibodies (mAbs). GPC3 was partially cleaved between Arg358 and Ser359, generating a C-terminal 30-kDa fragment and an N-terminal 40-kDa fragment. All mAbs that induced antibody-dependent cellular cytotoxicity (ADCC) and/or complement-dependent cytotoxicity (CDC) against cells expressing GPC3 recognized the 30-kDa fragment, indicating that the C-terminal region of GPC3 serves as an epitope for mAb with ADCC and/or CDC inducing activities. Chimeric mAbs with Fc replaced by human IgG1 were created from GC33, one of the mAbs that reacted with the C-terminal 30-kDa fragment. Chimeric GC33 induced not only ADCC against GPC3-positive human HCC cells but also was efficacious against the Huh-7 human HCC xenograft. Thus, mAbs against the C-terminal 30-kDa fragment such as GC33 are useful in therapy targeting HCC.

In vivo effects of mutant RHOA on tumor formation in an orthotopic inoculation model.

Ras homolog family member A (RHOA) mutations are driver genes in diffuse­type gastric cancers (DGCs), and we previously revealed that RHOA mutations contribute to cancer cell survival and cell migration through their dominant negative effect on Rho­associated kinase (ROCK) signaling in vitro. However, how RHOA mutations contribute to DGC development in vivo is poorly understood. In the present study, the contribution of RHOA mutations to tumor morphology was investigated using an orthotopic xenograft model using the gastric cancer cell line MKN74, in which wild­type (WT) or mutated (Y42C and Y42S) RHOA had been introduced. When we conducted RNA sequencing to distinguish between the genes expressed in human tumor tissues from those in mouse stroma, the expression profiles of the tumors were clearly divided into a Y42C/Y42S group and a mock/WT group. Through gene set enrichment analysis, it was revealed that inflammation­ and hypoxia­related pathways were enriched in the mock/WT tumors; however, cell metabolism­ and cell cycle­related pathways such as Myc, E2F, oxidative phosphorylation and G2M checkpoint were enriched in the Y42C/Y42S tumors. In addition, the gene set related to ROCK signaling inhibition was enriched in the RHOA­mutated group, which indicated that a series of events are related to ROCK inhibition induced by RHOA mutations. Histopathological analysis revealed that small tumor nests were more frequent in RHOA mutants than in the mock or WT group. In addition, increased blood vessel formation and infiltration of macrophages within the tumor mass were observed in the RHOA mutants. Furthermore, unlike mock/WT, the RHOA­mutated tumor cells had little antitumor host reaction in the invasive front, which is similar to the pattern of mucosal invasion in clinical RHOA­mutated DGC. These transcriptome and pathological analyses revealed that mutated RHOA functionally contributes to the acquisition of DGC features, which will accelerate our understanding of the contribution of RHOA mutations in DGC biology and the development of further therapeutic strategies.

Generation of an anti-desmoglein 3 antibody without pathogenic activity of pemphigus vulgaris for therapeutic application to squamous cell carcinoma.

It is ideal for the target antigen of a cytotoxic therapeutic antibody against cancer to be cancer-specific, but such antigens are rare. Thus an alternative strategy for target selection is necessary. Desmoglein 3 (DSG3) is highly expressed in lung squamous cell carcinoma, while it is well-known that anti-DSG3 antibodies cause pemphigus vulgaris, an autoimmune disease. We evaluated DSG3 as a novel target by selecting an epitope that exerts efficacy against cancer with no pathogenic effects in normal tissues. Pathogenic anti-DSG3 antibodies induce skin blisters by inhibiting the cell-cell interaction in a Ca2+-dependent manner. We screened anti-DSG3 antibodies that bind DGS3 independent of Ca2+ and have high antibody-dependent cell cytotoxicity (ADCC) activity against DSG3-expressing cells. These selected antibodies did not inhibit cell-cell interaction and showed ADCC activity against squamous cell carcinoma cell lines. Furthermore, one of the DSG3 antibodies showed anti-tumour activity in tumour mouse models but did not induce adverse effects such as blister formation in the skin. Thus it was possible to generate an antibody against DSG3 by using an appropriate epitope that retained efficacy with no pathogenicity. This approach of epitope selection may expand the variety of druggable target molecules.

DGC-specific RHOA mutations maintained cancer cell survival and promoted cell migration via ROCK inactivation.

RHOA missense mutations exist specifically in diffuse type gastric cancers (DGC) and are considered one of the DGC driver genes, but it is not fully understood how RHOA mutations contribute to DGC development. Here we examined how RHOA mutations affect cancer cell survival and cell motility. We revealed that cell survival was maintained by specific mutation sites, namely G17, Y42, and L57. Because these functional mutations suppressed MLC2 phosphorylation and actin stress fiber formation, we realized they act in a dominant-negative fashion against the ROCK pathway. Through the same inactivating mechanism that maintained cell survival, RHOA mutations also increased cell migration activity. Cell survival and migration studies on CLDN18-ARHGAP (CLG) fusions, which are known to be mutually exclusive to RHOA mutations, showed that CLG fusions complemented cell survival under RHOA knockdown condition and also induced cell migration. Site-directed mutagenesis analysis revealed the importance of the GAP domain and indicated that CLG fusions maintained RHOA in the inactive form. Taken together, these findings show that the inactivation of ROCK would be a key step in DGC development, so ROCK activation might provide novel therapeutic opportunities.

Generation and characterization of monoclonal antibodies against human LGR6.

Leucine-rich repeat-containing G protein-coupled receptor 6 (LGR6) is a seven-pass transmembrane protein known to be a marker of stem cells in several organs. To deepen our understanding of the cell biology of LGR6-positive cells, including stem cells, we generated monoclonal antibodies (mAbs) against human LGR6. DNA immunization followed by whole-cell immunization with LGR6-expressing transfectants was performed to obtain mAbs that recognized the native form of LGR6. Hybridomas were screened by flow cytometry using LGR6-transfected cells. Because the molecules of LGR4, LGR5, and LGR6 are 50% homologous at the amino acid level, specificity of the mAbs was confirmed by transfectants expressing LGR4, LGR5, or LGR6. Three LGR6-specific mAbs were generated. Two of the three mAbs (designated 43A6 and 43D10) recognized the large N-terminal extracellular domain of LGR6, and competitively blocked the binding of R-spondin 1, which is known to be the ligand for LGR6. The other mAb, 43A25, recognized the seven-pass transmembrane domain of LGR6, and was able to be used for immunoblot analysis. In addition, mAbs 43A6 and 43D10 detected endogenous expression of LGR6 in cancer cell lines. We expect that our mAbs will contribute to widening our understanding of LGR6-positive cells in humans.

Identification of soluble NH2-terminal fragment of glypican-3 as a serological marker for early-stage hepatocellular carcinoma.

For detection of hepatocellular carcinoma (HCC) in patients with liver cirrhosis, serum alpha-fetoprotein has been widely used, but its sensitivity has not been satisfactory, especially in small, well-differentiated HCC, and complementary serum marker has been clinically required. Glypican-3 (GPC3), a heparan sulfate proteoglycan anchored to the plasma membrane, is a good candidate marker of HCC because it is an oncofetal protein overexpressed in HCC at both the mRNA and protein levels. In this study, we demonstrated that its NH(2)-terminal portion [soluble GPC3 (sGPC3)] is cleaved between Arg(358) and Ser(359) of GPC3 and that sGPC3 can be specifically detected in the sera of patients with HCC. Serum levels of sGPC3 were 4.84 +/- 8.91 ng/ml in HCC, significantly higher than the levels seen in liver cirrhosis (1.09 +/- 0.74 ng/ml; P < 0.01) and healthy controls (0.65 +/- 0.32 ng/ml; P < 0.001). In well- or moderately-differentiated HCC, sGPC3 was superior to alpha-fetoprotein in sensitivity, and a combination measurement of both markers improved overall sensitivity from 50% to 72%. These results indicate that sGPC3 is a novel serological marker essential for the early detection of HCC.

Effective screening method of agonistic diabodies based on autocrine growth.

Agonistic diabodies that mimic the function of natural ligands are expected to increase the value of therapeutic antibodies. We have developed a method that detects agonistic diabodies based on their ability to transduce growth signals through receptors, thereby permitting cytokine-independent growth of BaF/3-derived cytokine-dependent cells. Retrovirus-mediated expression of the diabody in cytokine-dependent cells was followed by selection of clones for growth in the absence of cytokine. A diabody library derived from splenocytes of human Mpl immunized mice was constructed. Infection of cells with viral particles led to the isolation of over 500 autonomously growing clones whose cultured supernatants showed agonistic activities against Mpl. Genome-integrated diabodies were cloned; representative clone AB317 showed agonistic activities as potent as a natural ligand and cross-reactive against mouse Mpl.

Anti-glypican 3 antibody as a potential antitumor agent for human liver cancer.

Human glypican 3 (GPC3) is preferentially expressed in the tumor tissues of liver cancer patients. In this study, we obtained a monoclonal antibody (mAb) against the COOH-terminal part of GPC3, which induced antibody-dependent cellular cytotoxicity (ADCC). The mAb, designated GC33, exhibited marked tumor growth inhibition of s.c. transplanted Hep G2 and HuH-7 xenografts that expressed GPC3 but did not inhibit growth of the SK-HEP-1 that was negative for GPC3. GC33 was efficacious even in an orthotopic model; it markedly reduced the blood alpha-fetoprotein levels of mice intrahepatically transplanted with Hep G2 cells. Humanized GC33 (hGC33) was as efficacious as GC33 against the Hep G2 xenograft, but hGC33 lacking carbohydrate moieties caused neither ADCC nor tumor growth inhibition. Depletion of CD56+ cells from human peripheral blood mononuclear cells markedly abrogated the ADCC caused by hGC33. The results show that the antitumor activity of hGC33 is mainly attributable to ADCC, and in human, natural killer cell-mediated ADCC is one possible mechanism of the antitumor effects by GC33. hGC33 will provide a novel treatment option for liver cancer patients with GPC3-positive tumors.

A novel therapeutic approach for thrombocytopenia by minibody agonist of the thrombopoietin receptor.

Antibodies have brought valuable therapeutics in the clinical treatment of various diseases without serious adverse effects through their intrinsic features such as specific binding to the target antigen with high affinity, clinical safety as serum proteins, and long half-life. Agonist antibodies, furthermore, could be expected to maximize the value of therapeutic antibodies. Indeed, several IgG/IgM antibodies have been reported to induce cellular growth/differentiation and apoptosis. These agonist antibodies, however, should be further improved to exert more potent biologic activities and appropriate serum half-life depending upon the disease indications. Here, we report that IgG antibodies against the thrombopoietin receptor (Mpl), which have an absence or very weak agonist activity, can be engineered to be agonist minibodies, which include diabody or sc(Fv)2 as potent as natural ligand. Through this technological development, minibodies have been successfully constructed to bind and activate 2 types of dysfunctional mutant Mpls that cause congenital amegakaryocytic thrombocytopenia (CAMT). This drastic conversion of biologic activities by designing minibodies can be widely applicable to generate agonist minibodies for clinical application, which will constitute a new paradigm in antibody-based therapeutics.

betaARK1 inhibition improves survival in a mouse model of heart failure induced by myocardial infarction.

Heart failure (HF) is characterized by abnormalities in beta-adrenergic receptor (betaAR) signaling, including an increase in betaAR kinase 1 (betaARK1) levels and activity. Gene therapy using a peptide inhibitor of betaARK1 (betaARKct) in infarcted rabbit hearts has improved compromised cardiac function. To determine whether betaARK1 inhibition improves survival in a mouse model of HF induced by myocardial infarction (MI), we studied wild-type (WT) and transgenic (TG) mice overexpressing betaARKct following MI. There was no difference in infarct size. Survival of WT mice with MI was 25% at 26 weeks. In contrast, 92% of betaARKct TG mice with MI survived (P = 0.01). betaARKct TG mice with MI at 8 weeks showed significantly higher fractional shortening compared with WT mice with MI (25.1 +/- 2.7% versus 14.2 +/- 1.0%; P < 0.05). Moreover, the biochemical betaAR abnormalities in WT mice with MI were prevented in betaARKct TG mice with MI. In conclusion, betaARK1 inhibition results in a marked increase in survival and improved cardiac function in a mouse model of HF induced by MI.

Novel heat shock protein HspQ stimulates the degradation of mutant DnaA protein in Escherichia coli.

Escherichia coli DnaA protein initiates chromosomal replication and is an important regulatory target during the replication cycle. In this study, a suppressor mutation isolated by transposon mutagenesis was found to allow growth of the temperature-sensitive dnaA508 and dnaA167 mutants at 40 degrees C. The suppressor consists of a transposon insertion in a previously annotated ORF, here termed hspQ, a novel heat shock gene whose promoter is recognized by the major heat shock sigma factor sigma32. Expression of hspQ on a pBR322 derivative inhibits growth of the dnaA508 and dnaA167 mutants at 30 degrees C, whereas growth of dnaA46 and other dnaA mutants is insensitive to changes in the level of hspQ. Cellular DnaA508 protein is degraded rapidly at elevated temperature, but hspQ disruption impedes this process. In contrast, DnaA46 protein is rapidly degraded in an hspQ-independent manner. Gel-filtration and chemical cross-linking experiments suggest that HspQ forms a stable homodimer in solution and can form homomultimers consisting of about four monomers. Heat-shock induced proteases such as Clp contain homomultimers of subunit proteins. We propose that HspQ is a new factor involved in the quality control of proteins and that it functions by excluding denatured proteins.