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Degradation of mouse invariant chain: roles of cathepsins S and D and the influence of major histocompatibility complex polymorphism.

Villadangos JA, Riese RJ, Peters C, Chapman HA, Ploegh HL - J. Exp. Med. (1997)

Bottom Line: Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules.Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding.These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II-peptide complexes and, second, that most class II-associated peptides can still be generated in cells treated with inhibitors of cysteine proteases.

View Article: PubMed Central - PubMed

Affiliation: Center for Cancer Research, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules. Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding. The cellular location of Ii degradation, as well as the enzymology of this event, are important in determining the sets of antigenic peptides that will bind to class II molecules. Here, we show that the cysteine protease cathepsin S acts in a concerted fashion with other cysteine and noncysteine proteases to degrade mouse Ii in a stepwise fashion. Inactivation of cysteine proteases results in incomplete degradation of Ii, but the extent to which peptide loading is blocked by such treatment varies widely among MHC class II allelic products. These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II-peptide complexes and, second, that most class II-associated peptides can still be generated in cells treated with inhibitors of cysteine proteases. Surprisingly, maturation of MHC class II in mice deficient in cathepsin D is unaffected, showing that this major aspartyl protease is not involved in degradation of Ii or in generation of the bulk of antigenic peptides.

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Conservation of potential cleavage sites for Cat S in human  and mouse Ii. Residues 77–111 of human Ii (single-letter code), with the  corresponding mouse sequence. The bars indicate the CLIP regions (15,  78). The shaded bar indicates the residues of human CLIP that interact directly with the peptide-binding cleft of HLA-DR3 (5). The arrows indicate the potential target sites for Cat S as proposed in reference 37.
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Figure 5: Conservation of potential cleavage sites for Cat S in human and mouse Ii. Residues 77–111 of human Ii (single-letter code), with the corresponding mouse sequence. The bars indicate the CLIP regions (15, 78). The shaded bar indicates the residues of human CLIP that interact directly with the peptide-binding cleft of HLA-DR3 (5). The arrows indicate the potential target sites for Cat S as proposed in reference 37.

Mentions: The process of degradation of Ii that we have described occurs in stages, through sequential cleavages at specific sites. This model implies that either the structure of each substrate prevents the use of cleavage sites downstream in the degradation pathway, as supported by experiments in vitro (59), or that the different protease activities are compartmentalized. This could be achieved physically (i.e., in the extreme version, each protease would be present and active at a different compartment of the endocytic route) or functionally, so that different proteases accompanying the newly synthesized αβ–Ii complexes would be activated sequentially. The compartmentalization of the stages of Ii degradation is crucial because αβ–CLIP can escape to the cell surface (15–17, 60). Therefore, cleavage of LIP10 NH2 terminally of CLIP by Cat S probably takes place in compartments rich in antigenic peptides and H-2M such as CIIV/MIIC, in which formation of most mature αβ–peptide complexes occurs (39, 40, 61–66). We emphasize the importance of degrading Ii in a staged fashion as a means to ensure proper loading of antigenic peptides, because the enzymes that cleave Ii and the intermediates that they generate have been conserved during evolution, and appear similar in mouse splenocytes and in human B cells (32–34, 37, 39, 40, 67). Riese et al. (37) proposed two potential cleavage sites for Cat S in human Ii NH2 terminally of CLIP, after residues arg78 and lys80, both of which are conserved in mouse Ii (Fig. 5). Crystallography of HLA-DR1–CLIP complexes has shown that the NH2-terminal end of CLIP protrudes from the peptide binding region of the αβ heterodimer (5). Therefore, the region of Ii that contains the potential cleavage sites for Cat S may be located in an exposed loop in the αβIi trimer.


Degradation of mouse invariant chain: roles of cathepsins S and D and the influence of major histocompatibility complex polymorphism.

Villadangos JA, Riese RJ, Peters C, Chapman HA, Ploegh HL - J. Exp. Med. (1997)

Conservation of potential cleavage sites for Cat S in human  and mouse Ii. Residues 77–111 of human Ii (single-letter code), with the  corresponding mouse sequence. The bars indicate the CLIP regions (15,  78). The shaded bar indicates the residues of human CLIP that interact directly with the peptide-binding cleft of HLA-DR3 (5). The arrows indicate the potential target sites for Cat S as proposed in reference 37.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2199027&req=5

Figure 5: Conservation of potential cleavage sites for Cat S in human and mouse Ii. Residues 77–111 of human Ii (single-letter code), with the corresponding mouse sequence. The bars indicate the CLIP regions (15, 78). The shaded bar indicates the residues of human CLIP that interact directly with the peptide-binding cleft of HLA-DR3 (5). The arrows indicate the potential target sites for Cat S as proposed in reference 37.
Mentions: The process of degradation of Ii that we have described occurs in stages, through sequential cleavages at specific sites. This model implies that either the structure of each substrate prevents the use of cleavage sites downstream in the degradation pathway, as supported by experiments in vitro (59), or that the different protease activities are compartmentalized. This could be achieved physically (i.e., in the extreme version, each protease would be present and active at a different compartment of the endocytic route) or functionally, so that different proteases accompanying the newly synthesized αβ–Ii complexes would be activated sequentially. The compartmentalization of the stages of Ii degradation is crucial because αβ–CLIP can escape to the cell surface (15–17, 60). Therefore, cleavage of LIP10 NH2 terminally of CLIP by Cat S probably takes place in compartments rich in antigenic peptides and H-2M such as CIIV/MIIC, in which formation of most mature αβ–peptide complexes occurs (39, 40, 61–66). We emphasize the importance of degrading Ii in a staged fashion as a means to ensure proper loading of antigenic peptides, because the enzymes that cleave Ii and the intermediates that they generate have been conserved during evolution, and appear similar in mouse splenocytes and in human B cells (32–34, 37, 39, 40, 67). Riese et al. (37) proposed two potential cleavage sites for Cat S in human Ii NH2 terminally of CLIP, after residues arg78 and lys80, both of which are conserved in mouse Ii (Fig. 5). Crystallography of HLA-DR1–CLIP complexes has shown that the NH2-terminal end of CLIP protrudes from the peptide binding region of the αβ heterodimer (5). Therefore, the region of Ii that contains the potential cleavage sites for Cat S may be located in an exposed loop in the αβIi trimer.

Bottom Line: Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules.Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding.These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II-peptide complexes and, second, that most class II-associated peptides can still be generated in cells treated with inhibitors of cysteine proteases.

View Article: PubMed Central - PubMed

Affiliation: Center for Cancer Research, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules. Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding. The cellular location of Ii degradation, as well as the enzymology of this event, are important in determining the sets of antigenic peptides that will bind to class II molecules. Here, we show that the cysteine protease cathepsin S acts in a concerted fashion with other cysteine and noncysteine proteases to degrade mouse Ii in a stepwise fashion. Inactivation of cysteine proteases results in incomplete degradation of Ii, but the extent to which peptide loading is blocked by such treatment varies widely among MHC class II allelic products. These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II-peptide complexes and, second, that most class II-associated peptides can still be generated in cells treated with inhibitors of cysteine proteases. Surprisingly, maturation of MHC class II in mice deficient in cathepsin D is unaffected, showing that this major aspartyl protease is not involved in degradation of Ii or in generation of the bulk of antigenic peptides.

Show MeSH
Related in: MedlinePlus