Molecular cloning, sequencing and expression of obese gene in the Chinese

OBJECTIVE

To construct the human obese (ob) cDNA clone in the Chinese, and analyze the expression of the ob gene in adipose tissue of obese, non-obese subjects and nooinsulin-dependent diabetes mellitus (NIDDM) Chinese patients.

METHODS

A ob cDNA clone was isolated by reverse transcription polymerase chain reaction (RT-PCR). Four groups of Chinese subjects participated in the study: 1) 12 obese subjects [body mass index (BMI): 28.5 +/- 2.3 kg/m2]; 2) 11 non-obese subjects (BMI: 21.0 +/- 1.5 kg/m2); 3) 8 obese NIDDM patients (BMI: 27.0 +/- 1.4 kg/m2); 4) 11 non-obese NIDDM patients (BMI: 21.2 +/- 1.4 kg/m2). The expression of ob gene mRNA in abdominal subcutaneous adipose tissue was examined using RNA dot blot hybridization with a digoxigenin-labeled human ob cDNA probe. The hybridized signals were quantitated by densitometry.

RESULTS

A full human ob cDNA fragment which included a glutamine codon at +49 was obtained. A base substitution (A to G) in the coding region at position 287 was found, resulting in a glutamine being replaced by an arginine. Expression of the ob gene was significantly higher in Chinese obese subjects compared to non-obese ones (P < 0.05), and positively correlated with the BMI. No significant difference in the amount of ob mRNA was detected between non-diabetic and diabetic groups at the same BMI level.

CONCLUSIONS

We constructed a full length human ob cDNA clone. The expression of the ob gene was significantly higher in Chinese obese subjects than in non-obese ones. The metabolic and hormonal changes associated with NIDDM are not the main factors regulating the expression of the ob gene.

Broadening the Aldolase Catalytic Antibody Repertoire by Combining Reactive Immunization and Transition State Theory: New Enantio- and Diastereoselectivities

Nine efficient aldolase antibodies were generated by using hapten 1. This hapten unites reactive immunization and the transition state analogue approach in a single molecule. Characterization of two of these antibodies reveals that they are highly proficient (up to 1000-fold better than any other antibody catalyst) and enantioselective catalysts for aldol and retro-aldol reactions and exhibit enantio- and diastereoselectivities opposite to that of antibody 38C2.

[An outline on the early pharmacetical development before Galen]

The period before Galen (including his time) is known as ancient herbal period of pharmacy, which is an important component in the developing history of pharmacy. Some characteristics and rules about early pharmacy development are reflectd from the origin of pharmacy, Babylonian - Assyrian pharmacy, ancient Egyptian pharmacy, ancient Greek pharmacy and Rome pharmacy in different periods and areas.

Chlamydia inhibits interferon gamma-inducible major histocompatibility complex class II expression by degradation of upstream stimulatory factor 1

We report that chlamydiae, which are obligate intracellular bacterial pathogens, can inhibit interferon (IFN)-gamma-inducible major histocompatibility complex (MHC) class II expression. However, the IFN-gamma-induced IFN regulatory factor-1 (IRF-1) and intercellular adhesion molecule 1 (ICAM-1) expression is not affected, suggesting that chlamydia may selectively target the IFN-gamma signaling pathways required for MHC class II expression. Chlamydial inhibition of MHC class II expression is correlated with degradation of upstream stimulatory factor (USF)-1, a constitutively and ubiquitously expressed transcription factor required for IFN-gamma induction of class II transactivator (CIITA) but not of IRF-1 and ICAM-1. CIITA is an obligate mediator of IFN-gamma-inducible MHC class II expression. Thus, diminished CIITA expression as a result of USF-1 degradation may account for the suppression of the IFN-gamma-inducible MHC class II in chlamydia-infected cells. These results reveal a novel immune evasion strategy used by the intracellular bacterial pathogen chlamydia that improves our understanding of the molecular basis of pathogenesis.

Immunity to Chlamydia trachomatis mouse pneumonitis induced by vaccination with live organisms correlates with early granulocyte-macrophage colony-stimulating factor and interleukin-12 production and with dendritic cell-like maturation

As is true for other intracellular pathogens, immunization with live Chlamydia trachomatis generally induces stronger protective immunity than does immunization with inactivated organism. To investigate the basis for such a difference, we studied immune responses in BALB/c mice immunized with viable or UV-killed C. trachomatis mouse pneumonitis (MoPn). Strong, acquired resistance to C. trachomatis infection was elicited by immunization with viable but not dead organisms. Immunization with viable organisms induced high levels of antigen-specific delayed-type hypersensitivity (DTH), gamma interferon production, and immunoglobulin A (IgA) responses. Immunization with inactivated MoPn mainly induced interleukin-10 (IL-10) production and IgG1 antibody without IgA or DTH responses. Analysis of local early cytokine and cellular events at days 3, 5, and 7 after peritoneal cavity immunization showed that high levels of granulocyte-macrophage colony-stimulating factor and IL-12 were detected with viable but not inactivated organisms. Furthermore, enrichment of a dendritic cell (DC)-like population was detected in the peritoneal cavity only among mice immunized with viable organisms. The results suggest that early differences in inducing proinflammatory cytokines and activation and differentiation of DCs may be the key mechanism underlying the difference between viable and inactivated organisms in inducing active immunity to C. trachomatis infection.

Interleukin-12 production is required for chlamydial antigen-pulsed dendritic cells to induce protection against live Chlamydia trachomatis infection

Immunization with dendritic cells pulsed ex vivo with antigens has been successfully used to elicit primary antigen-specific immune responses. We report that mouse bone marrow-derived dendritic cells pulsed with inactivated chlamydial organisms induced strong protection against live chlamydial infection in a mouse lung infection model. Either the dendritic cells or chlamydial organisms alone or macrophages similarly pulsed with chlamydial organisms failed to induce any significant protection. These observations suggest that dendritic cells can efficiently process and present chlamydial antigens to naive T cells in vivo. Mice immunized with the chlamydia-pulsed dendritic cells preferentially developed a Th1 cell-dominant response while mice immunized with the other immunogens did not, suggesting a correlation between a Th1 cell-dominant response and protection against chlamydial infection. We further found that dendritic cells produced a large amount of interleukin 12 (IL-12) upon ex vivo pulsing with inactivated chlamydial organisms, which may allow the dendritic cells to direct a Th1 cell-dominant response. Dendritic cells from mice deficient in the IL-12 p40 gene failed to produce IL-12 after a similar ex vivo pulse with chlamydial organisms, and more importantly, immunization with these dendritic cells failed to induce a Th1 cell-dominant response and did not induce strong protection against chlamydial infection. Thus, the ability of dendritic cells to efficiently process and present chlamydial antigens and to produce IL-12 upon chlamydial-organism stimulation are both required for the induction of protection against chlamydial infection. This information may be useful for the further design of effective chlamydial vaccines.

Calcineurin B- and calmodulin-binding preferences identified with phage-displayed peptide libraries

Calcineurin B (CnB) and calmodulin (CaM) are two structurally similar but functionally distinct 'EF-hand' Ca2+-binding proteins. CnB is the regulatory subunit of the CaM-stimulated protein phosphatase, calcineurin. CaM is a unique multifunctional protein that interacts with and modulates the activity of many target proteins. CnB and CaM are both required for the full activation of the phosphatase activity of calcineurin and are not interchangeable. The two proteins recognize distinct binding sites on calcineurin A subunit (CnA) and perform different functions. Phage-displayed peptide libraries (pIII and pVIII libraries) were screened with CnB and CaM to isolate peptides that could then be compared to determine if there were binding preferences of the two proteins. The Ca2+-dependent binding of phage-displayed peptides to CnB and CaM is specifically blocked by synthetic peptides derived from the CnB-binding domain of CnA and the CaM-binding domain of myosin light chain kinase respectively. Both CnB- and CaM-binding peptides have a high content of tryptophan and leucine, but CnB-binding peptides are more hydrophobic than CaM-binding peptides. CnB-binding peptides are negatively charged with clusters of hydrophobic residues rich in phenylalanine, whereas the CaM-binding peptides are positively charged and often contain an Arg/Lys-Trp motif. The binding preferences identified with peptide libraries are consistent with the features of the CnB-binding domains of all CnA isoforms and the CaM-binding domains of CaM targets.

The atherogenic effects of chlamydia are dependent on serum cholesterol and specific to Chlamydia pneumoniae

Epidemiological investigations have linked Chlamydia pneumoniae infection to atherosclerosis. It is not clear, however, whether C. pneumoniae infection plays a causal role in the development of atherosclerosis. Mice with low-density lipoprotein receptor deficiency were induced to develop atherosclerotic lesions in aorta with a cholesterol-enriched diet that increased serum cholesterol by two- to threefold. Using this mouse model, we found that the chlamydial infection alone with either the C. pneumoniae AR39 or the C. trachomatis MoPn strain failed to induce any significant atherosclerotic lesions in aorta over a period of nine months. However, in the presence of a high-cholesterol diet, infection with the C. pneumoniae AR39 strain significantly exacerbated the hypercholesterolemia-induced atherosclerosis, demonstrating that a hypercholesterolemic condition is required for the C. pneumoniae to aggravate the development of atherosclerosis. Although both AR39 and MoPn antigens were detected in aorta of mice infected with the corresponding strains, only mice infected with the C. pneumoniae strain AR39 displayed enhanced atherosclerotic lesions, suggesting that the C. pneumoniae species may possess a unique atherogenic property. This study may provide a model for further understanding the mechanisms of C. pneumoniae atherogenesis and evaluating chlamydial intervention strategies for preventing the advancement of atherosclerotic lesions enhanced by C. pneumoniae infection.

[An experimental study on bone potentials at fracture site with axial interfragmental gradient compression]

OBJECTIVE

To evaluate the mechanism of stress generated potentials in compressive fixation leading to the primary healing.

METHODS

Eighteen fresh sheep tibiae were divided equally into three groups, and all specimen were fractured artificially and imposed "prestressing" at fracture sites with tension device and plate, meanwhile stainless wire tension band was used in the place opposite to plate. The interfragmental pressure was exerted when applied plates, which were 98.62 +/- 19.74 N, 137.93 +/- 8.13 N and 169.20 +/- 11.33 N. Bone potentials at fracture sites were recorded with reference to the potential measurement system by Friedenberg and Brighton (1966).

RESULTS

There was no significant statistical difference in bone potentials when made comparison between before and after compression fixation in each group and among groups of different axial interfragmental gradient compressions.

CONCLUSION

The mechanism of stress generated potentials may not exist in primary healing with compressive fixation.

Contribution of the absence of aspartic acid at position 57 of the HLA-DQ beta chain to predisposition to insulin-dependent diabetes mellitus in a southern Chinese population

OBJECTIVE

To investigate the association of the absence or presence of aspartic acid at position 57 of the HLA-DQ beta chain (NA or A) with susceptibility or resistance to insulin-dependent diabetes mellitus (IDDM) in a Southern Chinese population.

METHODS

Sixty-nine IDDM patients and 47 healthy controls in a Southern Chinese population were HLA-DQB1 genotyped by one-step sequence specific polymerase chain reaction (ssPCR).

RESULTS

The frequencies of NA and A were 64.5% and 35.5% in the IDDM patients, and 40.4% and 59.6% in the control subjects respectively (RR for NA was 2.68, P < 0.01). The frequencies of NA/NA, NA/A and A/A phenotypes were 47.8%, 33.3% and 18.8% in the IDDM patients, and 31.9%, 17.0% and 51.1% in the controls respectively (P < 0.01). The frequency of A/A phenotype was significantly lower in the IDDM patients than in the control subjects (RR = 0.22, P < 0.01). DQB1* 0302 and DQB1* 0201 were more frequent in IDDM patients than in control subjects. The younger the age of IDDM onset, the higher the allele frequencies of DQB1* 0201 and DQB1* 0302.

CONCLUSION

The present study suggests that the NA confers the susceptibility to IDDM, while the A confers the protection against IDDM in patients of Southern Chinese origin. These associations are more clearcut in childhood-onset IDDM patients.

Protective effect of the angiotensin-converting enzyme inhibitor perindopril on diabetic glomerulopathy in streptozotocin-induced diabetic rats

OBJECTIVES

To evaluate the protective effect of the angiotensin-converting enzyme inhibitor perindopril on diabetic glomerulopathy in rats with experimentally induced diabetes and explore its possible mechanisms.

METHODS

Ninety-two adult male Wistar rats were randomly allocated into diabetes mellitus (DM), diabetes mellitus + perindopril (DMP) and control (C) groups. According to the duration of diabetes or observation (1, 3, 6 months), each group was randomly subdivided into DM1, DM3, DM6; DMP1, DMP3, DMP6; and C1, C3, C6 groups. Diabetes was induced by intraperitoneal injection of streptozotocin. The rats in the DMP groups received perindopril 1 mg.kg-1.d-1, through gastric intubation. Urinary protein excretion rate was determined by the method of Coomassie brilliant blue. Plasma renin activity, renal tissue renin activity, and plasma and renal tissue angiotensin II concentration were assayed by radioimmunoassay (RIA). Renal tissue total RNA was extracted by the Chomezymskis AGPC method. Renal angiotensinogen mRNA expression level was assessed by slot blot hybridization using a full length rat angiotensinogen cDNA probe labelled with 32P-dCTP and a random primer.

RESULTS

There was increased activity of the renin angiotensin system in diabetic rats. Perindopril decreased proteinuria and delayed the progression of glomerular basement membrane thickening. However, it did not reduce the expansion of the mesangial matrix (P < 0.05). Renin activity increased and angiotensin II concentration decreased significantly in both plasma and renal tissue in diabetes + perindopril groups (P < 0.05).

CONCLUSIONS

Perindopril may help prevent the progression of diabetic glomerulopathy, and the inhibition of renin angiotensin system activity may be a mechanism for this action.

[The study of biocompatibility of titanium in middle ear cave of guinea-pig]

In order to supply full objective basis about tympanoplasty with titanium, 30 middle ear caves of guinea-pigs were implanted titanium ring in accordance with different time groups. Both before and after titanium ring were implanted, the guinea-pigs were detected in biochemical criterion of blood, titanium content of the hair, the changing of auditory threshold, tissue structural of middle ear and inner ear by energy spectrometry, electro-physiology, microscopy and scanning electron microscopy. The results showed that function of hepatorenal and titanium content of hair haven't significant change between pre- and postimplantation of titanium ring. Only 10 days group, it appeared the rising of auditory threshold and inverting of cilia by ABR and scanning electron microscopy. In 10 days and 1 month groups, there were blood stain and inflammatory cells under mucosa. The results suggested that titanium is good biocompatibility and no toxic for organism and mucosa.

Inhibition of apoptosis in chlamydia-infected cells: blockade of mitochondrial cytochrome c release and caspase activation

We report that chlamydiae, which are obligate intracellular bacterial pathogens, possess a novel antiapoptotic mechanism. Chlamydia-infected host cells are profoundly resistant to apoptosis induced by a wide spectrum of proapoptotic stimuli including the kinase inhibitor staurosporine, the DNA-damaging agent etoposide, and several immunological apoptosis-inducing molecules such as tumor necrosis factor-alpha, Fas antibody, and granzyme B/perforin. The antiapoptotic activity was dependent on chlamydial but not host protein synthesis. These observations suggest that chlamydia may encode factors that interrupt many different host cell apoptotic pathways. We found that activation of the downstream caspase 3 and cleavage of poly (ADP-ribose) polymerase were inhibited in chlamydia-infected cells. Mitochondrial cytochrome c release into the cytosol induced by proapoptotic factors was also prevented by chlamydial infection. These observations suggest that chlamydial proteins may interrupt diverse apoptotic pathways by blocking mitochondrial cytochrome c release, a central step proposed to convert the upstream private pathways into an effector apoptotic pathway for amplification of downstream caspases. Thus, we have identified a chlamydial antiapoptosis mechanism(s) that will help define chlamydial pathogenesis and may also provide information about the central mechanisms regulating host cell apoptosis.

Immune versus natural selection: antibody aldolases with enzymic rates but broader scope

Structural and mechanistic studies show that when the selection criteria of the immune system are changed, catalytic antibodies that have the efficiency of natural enzymes evolve, but the catalytic antibodies are much more accepting of a wide range of substrates. The catalytic antibodies were prepared by reactive immunization, a process whereby the selection criteria of the immune system are changed from simple binding to chemical reactivity. This process yielded aldolase catalytic antibodies that approximated the rate acceleration of the natural enzyme used in glycolysis. Unlike the natural enzyme, however, the antibody aldolases catalyzed a variety of aldol reactions and decarboxylations. The crystal structure of one of these antibodies identified the reactive lysine residue that was selected in the immunization process. This lysine is deeply buried in a hydrophobic pocket at the base of the binding site, thereby accounting for its perturbed pKa.

Production, specificity, and functionality of monoclonal antibodies to specific peptide-major histocompatibility complex class II complexes formed by processing of exogenous protein

Several unanswered questions in T cell immunobiology relating to intracellular processing or in vivo antigen presentation could be approached if convenient, specific, and sensitive reagents were available for detecting the peptide-major histocompatibility complex (MHC) class I or class II ligands recognized by alphabeta T cell receptors. For this reason, we have developed a method using homogeneously loaded peptide-MHC class II complexes to generate and select specific mAb reactive with these structures using hen egg lysozyme (HEL) and I-Ak as a model system. mAbs specific for either HEL-(46-61)-Ak or HEL-(116-129)-Ak have been isolated. They cross-react with a small subset of I-Ak molecules loaded with self peptides but can nonetheless be used for flow cytometry, immunoprecipitation, Western blotting, and intracellular immunofluorescence to detect specific HEL peptide-MHC class II complexes formed by either peptide exposure or natural processing of native HEL. An example of the utility of these reagents is provided herein by using one of the anti-HEL-(46-61)-Ak specific mAbs to visualize intracellular compartments where I-Ak is loaded with HEL-derived peptides early after antigen administration. Other uses, especially for in vivo tracking of specific ligand-bearing antigen-presenting cells, are discussed.

[Applied anatomy of osteo-periosteal flap pedicled with superior malleolar branch of anterior tibial artery]

In order to investigate the blood supply of osteo-periosteal flap of lateral inferior part of tibia, 40 lower limbs of adult cadavers were observed. The result showed that the superior malleolar branch was the biggest branch on the lateral inferior part of tibia and served as the main blood supply to the above area. It originated from the anterior tibial artery, 3.1 +/- 0.8 cm above the intermalleolar line. During its way to the anterior border of the tibia, it gave out the ascending and descending branches. The ascending branch was along the anterior border upward and anastomosed with the musculo-periosteol branch of the anterior tibial artery at the level of 6.3 +/- 1.3 cm above the intermalleolar line. The decending branch was anastomosed with the anterior medial malleolar artery. For the anastomosis between the superior malleolar branch with the peripheral vessels, the osteo-periosteol flap could be designed at the lateral side of the lower part of tibia in size of 8-10 cm x 4-6 cm. This was a new donor area of osteo-periosteol flap for repair of non-union of bone in lower end of tibia or arthrodesis of the ankle joint.

Antigen-unspecific B cells and lymphoid dendritic cells both show extensive surface expression of processed antigen-major histocompatibility complex class II complexes after soluble protein exposure in vivo or in vitro

Intravenous (i.v.) injection of high amounts of soluble proteins often results in the induction of antigen-specific tolerance or deviation to helper rather than inflammatory T cell immunity. It has been proposed that this outcome may be due to antigen presentation to T cells by a large cohort of poorly costimulatory or IL-12-deficient resting B cells lacking specific immunoglobulin receptors for the protein. However, previous studies using T cell activation in vitro to assess antigen display have failed to support this idea, showing evidence of specific peptide-major histocompatibility complex (MHC) class II ligand only on purified dendritic cells (DC) or antigen-specific B cells isolated from protein injected mice. Here we reexamine this question using a recently derived monoclonal antibody specific for the T cell receptor (TCR) ligand formed by the association of the 46-61 determinant of hen egg lysozyme (HEL) and the mouse MHC class II molecule I-Ak. In striking contrast to conclusions drawn from indirect T cell activation studies, this direct method of TCR ligand analysis shows that i.v. administration of HEL protein results in nearly all B cells in lymphoid tissues having substantial levels of HEL 46-61-Ak complexes on their surface. DC readily isolated from spleen also display this TCR ligand on their surface. Although the absolute number of displayed ligands is greater on such DC, the relative specific ligand expression compared to total MHC class II levels is similar or greater on B cells. These results demonstrate that in the absence of activating stimuli, both lymphoid DC and antigen-unspecific B cells present to a similar extent class II-associated peptides derived from soluble proteins in extracellular fluid. The numerical advantage of the TCR ligand-bearing B cells may permit them to interact first or more often with naive antigen-specific T cells, contributing to the induction of high-dose T cell tolerance or immune deviation.

Mapping epitopes of neutralizing monoclonal antibodies using phage random peptide libraries

Identification of protective determinants from microbial proteins is a necessary step in the rational design of subunit vaccines. We have previously used a synthetic peptide scan (Pepscan) assay to map a panel of eight neutralizing monoclonal antibodies (mAb; designated as C1.1 to C1.8) to a common motif sequence from Chlamydia trachomatis. In the present study, five of the eight mAbs were used to screen phage random peptide libraries. mAbs C1.1 and C1.3 selected a motif sequence of G-L-X-N-D from a pIII-based phage random peptide library and a motif sequence of G-X-X-N-D from a pVIII-based random peptide library while mAbs C1.6 to C1.8 failed to select recognizable motifs from either of the phage libraries. However, C1.6 to C1.8 bound to the same motif sequence displayed on phage when the appropriate conformational constraints were imposed onto the motif sequence. Thus the specificity of the mAbs identified on Pepscan assays correlates with the mAbs' dependence on local epitope constraints displayed on the phage surface.

Antigen presentation by MHC class II molecules: invariant chain function, protein trafficking, and the molecular basis of diverse determinant capture

Major histocompatibility complex class II molecules are heterodimeric integral membrane proteins whose primary function is the presentation of antigenic peptides derived from proteins entering the endocytic pathway to CD4+ T lymphocytes. To accomplish this physiologic function, class II molecules must assemble in the secretory pathway without undergoing irreversible ligand association at that site, traffic efficiently to the endocytic pathway, and productively interact with protein ligands in these organelles before their ultimate expression on the plasma membrane. Here we review our work describing how invariant chain promoters the assembly and transport process, the complex itinerary of class II-invariant chain complexes through the endocytic pathway, the role of large protein fragments as substrates for class II binding, and the existence of a second pathway for antigen capture by mature class II molecules that complements that involving newly synthesized dimers. We integrate these observations into a coherent model for the operation of a class II-dependent antigen processing and presentation system able to capture diverse antigenic determinants present in proteins of varying structure.

Related leucine-based cytoplasmic targeting signals in invariant chain and major histocompatibility complex class II molecules control endocytic presentation of distinct determinants in a single protein

Leucine-based signals in the cytoplasmic tail of invariant chain (Ii) control targeting of newly synthesized major histocompatibility complex class II molecules to the endocytic pathway for acquisition of antigenic peptides. Some protein determinants, however, do not require Ii for effective class II presentation, although endocytic processing is still necessary. Here we demonstrate that a dileucine-based signal in the cytoplasmic tail of the class II beta chain is critical for this Ii-independent presentation. Elimination or mutation of this signal reduces the rate of re-entry of mature surface class II molecules into the endocytic pathway. Antigen presentation controlled by this signal does not require newly synthesized class II molecules and appears to involve determinants requiring only limited proteolysis for exposure, whereas the opposite is true for li-dependent determinants. This demonstrates that related leucine-based trafficking signals in li and class II control the functional presentation of protein determinants with distinct processing requirements, suggesting that the peptide binding sites of newly synthesized versus mature class II molecules are made available for antigen binding in distinct endocytic compartments under the control of these homologous cytoplasmic signals. This permits capture of protein fragments produced optimally under distinct conditions of pH and proteolytic activity.

Evidence that binding site occupancy is necessary and sufficient for effective major histocompatibility complex (MHC) class II transport through the secretory pathway redefines the primary function of class II-associated invariant chain peptides (CLIP)

Invariant chain (Ii) associates with newly synthesized class II molecules in the endoplasmic reticulum (ER), an interaction that has been shown to interfere with peptide binding to class II molecules. The class II-associated invariant chain peptide (CLIP) region (residues 81-104) of Ii is believed to mediate this inhibition by engaging the binding domain of class II like an antigenic peptide. Together, these findings have given rise to a model in which CLIP association with the class II groove acts to prevent inappropriate presentation of peptides imported into the ER for association with major histocompatibility complex class I molecules. However, the properties of class II molecules synthesized by cells lacking coexpressed Ii are at least superficially inconsistent with this paradigm in that they do not show clear evidence of peptide acquisition. At the same time, we have previously shown the shortest form of Ii still containing CLIP to play an essential role in regulation of early class II molecule assembly and transport in the secretory pathway. Using covalent peptide technology, we now show that occupancy of the class II binding site in the ER regulates class II trafficking to the Golgi complex, an event that is the locus of the major defect in cells of Ii-deficient mice. These data argue that CLIP occupies the class II binding site, not to prevent interaction with short peptides meant for class I, but rather to maintain the structural integrity of class II molecules that are labile without engaged binding regions, and that would also associate with intact proteins in the ER if left unoccupied. By these means, CLIP occupancy of the class II binding site promotes effective export of useful class II molecules for endocytic peptide acquisition.