The MHC-encoded class I molecule, H-2Kk, demonstrates distinct requirements of assembly factors for cell surface expression: roles of TAP, Tapasin and beta2-microglobulin.
Major histocompatibility complex encoded class I (MHC-I) molecules display peptides derived from endogenous proteins for perusal by CD8+ T lymphocytes. H6, a mouse hepatoma cell line, expresses low levels of surface H-2Dd but not H-2Kk. Surface H-2Dd molecules are unstable and their levels, but not H-2Kk, are induced at 22 degrees C. Immunoprecipitation experiments revealed that H-2Kk, H-2Dd and beta2-microglobulin (beta2m) are expressed intracellularly; however no conformed MHC-I are present. Transcriptional profiling of factors required for MHC-I assembly demonstrated greatly reduced levels of the Transporter associated with antigen processing (Tap)2 subunit. The role of key assembly molecules in the MHC-I pathway was investigated by ectopic expression studies. Overexpression of beta2m enhanced surface H-2Dd, but not H-2Kk, levels whereas overexpression of TAP2 rescued surface H-2Kk, but not H-2Dd, levels. Interestingly, Tapasin plays a dual role: first, in quality control by reducing the induced surface expression of TAP2-mediated H-2Kk and beta2m-mediated H-2Dd levels. Secondly, Tapasin overexpression increases Tap2 transcripts and cooperates with TAPl or human beta2m to enhance surface H-2Kk expression; this synergy is TAP-dependent as demonstrated by infected cell protein 47 (ICP47) inhibition studies. Unlike the well studied H-2 MHC-I alleles, H-2Kb, H-2Db, H-2Kd and H-2Dd, a functional TAP is "essential" for H-2Kk cell surface expression.
Department of Biochemistry, Indian Institute of Science, Bangalore-560012, India.
S Jyothi
SJ
Prasanna
Dipankar
D
Nandi
D018528:ATP-Binding Cassette Transporters; D000595:Amino Acid Sequence; D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D006183:H-2 Antigens / Q000378:metabolism*; D064532:Histocompatibility Antigen H-2D; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D007136:Immunoglobulins / Q000378:metabolism*; D007371:Interferon-gamma / Q000378:metabolism; D026901:Membrane Transport Proteins; D051379:Mice; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D001613:beta 2-Microglobulin / Q000378:metabolism*
D018528:ATP-Binding Cassette Transporters; D017920:Antiporters; D006183:H-2 Antigens; C024755:H-2K(K) antigen; D064532:Histocompatibility Antigen H-2D; D015395:Histocompatibility Antigens Class I; D007136:Immunoglobulins; D026901:Membrane Transport Proteins; D001613:beta 2-Microglobulin; C107762:tapasin; C477327:transporter associated with antigen processing (TAP); D007371:Interferon-gamma