ORGANELLE MARKERS - Endoplasmic Reticulum

TAPI

TAP1, a 70kDa transmembrane protein, and TAP2 are two structurally related subunits of the transporter associated with antigen processing (TAP). The TAP complex is a member of the ATP binding cassette (ABC) family of transmembrane transporters. TAP1 and TAP2 each contain an N-terminal transmembrane region and Cterminal nucleotide binding domains (NBD). TAP1 and TAP2 form a complex in the endoplasmic reticulum (ER) membrane with the NBD oriented in the cytosol. The TAP transporter is an essential component of the MHC class I antigen presentation pathway by binding peptides in its cytosolic part and subsequently translocating the peptides into the lumen of the endoplasmic reticulum (ER) where assembly of MHC class I and pepide takes place. Assembly of MHC class I-ß2-micorglobulin (ß2-m) dimers in the ER involves 2 chaperones, calnexin which interacts with free class I heavy (H) chains and calreticulin which binds human class I-ß2 dimers prior to peptide loading. Calreticulin remains associated with at least a subset of class I molecules when they in turn bind to TAP . Polymorphic differences in MHC class I H chains can results in quantitative as well as qualitative differences in how they interact with peptide, ß2-m, calnexin, calreticulin, ERp57, TAP and Tapasin, a subunit of the TAP complex which binds to both TAP1 and MHC class I. Data obtained with Tapasin deletion mutants revealed that binding to TAP is mediated by the C-terminal region and that the N-terminal region is required to stabilize the MHC class I loading complex. The Tapasin gene is centromeric of HLA-DP locus between the HSET and HKE1.5 genes and within 500 kbp of the transporters associated with antigen processing, TAP1 and TAP2 genes. The localization of these genes within such a short distance of each other on the chromosome implies some regulatory or functional
significance.

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