Peptides Assemble with Class I Mhc Aided by Chaperone Molecules

Like other proteins, the a chain and ^-microglobulin components of the class I MHC molecule are synthesized on polysomes along the rough endoplasmic reticulum. Assembly of these components into a stable class I MHC molecular complex that can exit the RER requires the presence of a peptide in the binding groove of the class I molecule. The assembly process involves several steps and includes the participation of molecular chaperones, which facilitate the folding of polypeptides. The first molecular chaperone involved in class I MHC assembly is calnexin, a resident membrane protein of the endoplasmic reticulum. Calnexin associates with the free class I a chain and promotes its folding. When ^-microglobulin binds to the a chain, calnexin is released and the class I molecule associ-

TAP1

TAP2

Cytosol c

RER membrane

RER lumen

(b) Cytosol

Protein

ATP ADP + Class I MHC

(b) Cytosol

Protein

Rer Membrane

FIGURE 8-6

Generation of antigenic peptide-class I MHC complexes in the cytosolic pathway. (a) Schematic diagram of TAP, a heterodimer anchored in the membrane of the rough endoplasmic reticulum (RER). The two chains are encoded by TAP 1 and TAP2. The cytosolic domain in each TAP subunit contains an ATP-binding site, and peptide transport depends on the hydrolysis of ATP. (b) In the cytosol, association of LMP2, LMP7, and LMP10 (black spheres) with a protea-some changes its catalytic specificity to favor production ofpeptides that bind to class I MHC molecules. Within the RER membrane, a newly synthesized class I a chain associates with calnexin until ^-microglobulin binds to the a chain. The class I a chain/p2-microglobulin heterodimer then binds to calreticulin and the TAP-associated protein tapasin. When a peptide delivered by TAP is bound to the class I molecule, folding of MHC class I is complete and it is released from the RER and transported through the Golgi to the surface of the cell.

FIGURE 8-6

Generation of antigenic peptide-class I MHC complexes in the cytosolic pathway. (a) Schematic diagram of TAP, a heterodimer anchored in the membrane of the rough endoplasmic reticulum (RER). The two chains are encoded by TAP 1 and TAP2. The cytosolic domain in each TAP subunit contains an ATP-binding site, and peptide transport depends on the hydrolysis of ATP. (b) In the cytosol, association of LMP2, LMP7, and LMP10 (black spheres) with a protea-some changes its catalytic specificity to favor production ofpeptides that bind to class I MHC molecules. Within the RER membrane, a newly synthesized class I a chain associates with calnexin until ^-microglobulin binds to the a chain. The class I a chain/p2-microglobulin heterodimer then binds to calreticulin and the TAP-associated protein tapasin. When a peptide delivered by TAP is bound to the class I molecule, folding of MHC class I is complete and it is released from the RER and transported through the Golgi to the surface of the cell.

ates with the chaperone calreticulin and with tapasin. Tapasin (TAP-associated protein) brings the TAP transporter into proximity with the class I molecule and allows it to acquire an antigenic peptide (Figure 8-7). The physical association of the a chain-^-microglobulin

CLI N ICAL FOCUS

CLI N ICAL FOCUS

Deficiency in Transporters Associated with Antigen Presentation (TAP) Leads to a Diverse Disease Spectrum

A relatively ae dition known as bare lymphocyte syndrome, or BLS, has been recognized for more than 22 years. The lymphocytes in BLS patients express MHC molecules at below-normal levels and, in some cases, not at all. In type 1 BLS, a deficiency in MHC class I molecules exists; in type 2 BLS, expression of class II molecules is impaired. The pathogenesis of one type of BLS underscores the importance of the class I family of MHC molecules in their dual roles of preventing autoimmunity as well as defending against pathogens.

Defects in promoter sequences that preclude MHC gene transcription were found for some type 2 BLS cases, but in many instances the nature of the underlying defect is not known. A recent study has identified a group of patients with type 1 BLS due to defects in TAP 1 or TAP2 genes. Manifestations of the TAP deficiency were consistent in this patient group and define a unique disease. As described earlier in this chapter, TAP proteins are necessary for the loading of peptides onto class I molecules, a step that is essential for expression of class I MHC molecules on the cell surface. Lymphocytes in individuals with TAP deficiency express levels of class I molecules significantly lower than normal controls. Other cellular abnormalities include increased numbers of NK and 78 T cells, and decreased levels of CD8+ a^ T cells. As we shall see, the disease manifestations are reasonably well explained by these deviations in the levels of certain cells involved in immune function.

In early life the TAP-deficient individual suffers frequent bacterial infections of the upper respiratory tract, and in the second decade begins to have chronic infection of the lungs. It is thought that a post-nasal-drip syndrome common in younger patients promotes the bacterial lung infections in later life. Noteworthy is the absence of any severe viral infection, which is common in immunodeficiencies with T-cell involvement (see Chapter 19). Bronchiectasis (dilation of the bronchial tubes) often occurs and recurring infections can lead to lung damage that may be fatal. The most characteristic mark of the deficiency is the occurrence of necrotizing skin lesions on the extremities and the midface. These lesions ulcerate and may cause disfigurement (see figure). The skin lesions are probably due to activated NK cells and 78 T cells; NK

cells were isolated from biopsied skin from several patients, supporting this possibility. Normally, the activity of NK cells is limited through the action of killer-cell-inhibitory receptors (KIRs), which deliver a negative signal to the NK cell following interaction with class I molecules (see Chapter 14). The deficiency of class I molecules in TAP-related BLS patients explains the excessive activity of the NK cells. Activation of NK cells further explains the absence of severe virus infections, which are limited by NK and 78 cells.

The best treatment for the characteristic lung infections appears to be antibiotics and intravenous immunoglobulin. Attempts to limit the skin disease by immunosuppressive regimens, such as steroid treatment or cytotoxic agents, can lead to exacerbation of lesions and is therefore contraindicated. Mutations in the promoter region of TAP that preclude expression of the gene were found for several patients, suggesting the possibility of gene therapy, but the cellular distribution of class I is so widespread that it is not clear what cells would need to be corrected to alleviate all symptoms.

Necrotizing Lesions

Necrotizing granulomatous lesions in the midface of patient with TAP-deficiency syndrome. TAP deficiency leads to a condition with symptoms characteristic of autoimmunity, such as the skin lesions that appear on the extremities and the midface, as well as immunodeficiency that causes chronic sinusitis, leading to recurrent lung infection. [From S. D. Gadola et al, 1999, Lancet 354:1598, and 2000, Clinical and Experimental Immunology 121:173.]

Calnexin-associated class I MHC a chain

P2 microglobulin

Class I MHC a chain

Calnexin

Calnexin-associated class I MHC a chain

Calreticulin-tapasin-associated class I MHC molecule

P2 microglobulin

+ Peptides

Calreticulin-tapasin-associated class I MHC molecule

Tapasin Calreticulin

Class I MHC molecule

Class I MHC molecule

Tapasin Calreticulin

Calnexin

Tapasin Calreticulin

FIGURE 8-7

Assembly and stabilization of class I MHC molecules. Newly formed class I a chains associate with calnexin, a molecular chaperone, in the RER membrane. Subsequent binding to ^-microglobulin releases calnexin and allows binding to the chaperonin calreticulin and to tapasin, which is associated with the peptide transporter TAP. This association promotes binding of an antigenic peptide, which stabilizes the class I molecule-peptide complex, allowing its release from the RER.

heterodimer with the TAP protein (see Figure 8-6b) promotes peptide capture by the class I molecule before the peptides are exposed to the luminal environment of the RER. Peptides not bound by class I molecules are rapidly degraded. As a consequence of peptide binding, the class I molecule displays increased stability and can dissociate from calreticulin and tapasin, exit from the RER, and proceed to the cell surface via the Golgi. An additional chaperone protein, ERp57, has been observed in association with calnexin and calretic-ulin complexes. The precise role of this resident endoplasmic reticulum protein in the class I peptide assembly and loading process has not yet been defined, but it is thought to contribute to the formation of disulfide bonds during the maturation of class I chains. Because its role is not clearly defined, ERp57 is not shown in Figures 8-6 and 8-7.

Essentials of Human Physiology

Essentials of Human Physiology

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