Hydrophobic Interactions Are Key To Drive the Association of Tapasin with Peptide Transporter Subunit TAP2

Abstract

The transporter associated with Ag processing (TAP) translocates proteasomally derived cytosolic peptides into the endoplasmic reticulum. TAP is a central component of the peptide-loading complex (PLC), to which tapasin (TPN) recruits MHC class I (MHC I) and accessory chaperones. The PLC functions to facilitate and optimize MHC I–mediated Ag presentation. The heterodimeric peptide transporter consists of two homologous subunits, TAP1 and TAP2, each of which contains an N-terminal domain (N-domain) in addition to a conserved transmembrane (TM) core segment. Each N-domain binds to the TM region of a single TPN molecule, which recruits one MHC I molecule to TAP1 and/or TAP2. Although both N-domains act as TPN-docking sites, various studies suggest a functional asymmetry within the PLC resulting in greater significance of the TAP2/TPN interaction for MHC loading. In this study, we demonstrate that the leucine-rich hydrophobic sequence stretches (with the central leucine residues L20 and L66) in the first and second TM helix of TAP2 form a functional unit acting as a docking site for optimal TPN/MHC I recruitment, whereas three distinct highly conserved arginine and/or aspartate residues inside or flanking these TM helices are dispensable. Moreover, we show that the physical interaction between TAP2 and TPN is disrupted by benzene, a compound known to interfere with hydrophobic interactions, such as those between pairing leucine zippers. No such effects were observed for the TAP1/TAP2 interaction or the complex formation between TPN and MHC I. We propose that TAP/TPN complex formation is driven by hydrophobic interactions via leucine zipper–like motifs.

TAPBPR bridges UDP-glucose:glycoprotein glucosyltransferase 1 onto MHC class I to provide quality control in the antigen presentation pathway

“…Therefore, it seems plausible that tapasin and TAPBPR have evolved to function in distinct cellular environments. For tapasin, three regions have been identified that are essential for its localisation and function within the PLC: its transmembrane domain is responsible for its interaction with TAP (Petersen et al, 2005; Rufer et al, 2015); a free cysteine residue at position C95 is essential for its association with ERp57 (Dick et al, 2002; Peaper et al, 2005); and residues in the Ig domains interact with MHC class I (Turnquist et al, 2001; Turnquist et al, 2004, Dong et al, 2009). For TAPBPR, the only functional sites to be identified so far are those that are responsible for its interaction with MHC class I (Hermann et al, 2013), and as yet, no association partners that function with TAPBPR have been characterised.…”

Moving the Cellular Peptidome by Transporters

“…For rat TAP, however, this conserved aspartate in TAP2 can be exchanged to alanine without an effect on tapasin binding. In this case, leucine-rich areas in TM1 and TM2 of TAP2 seem to be important for tapasin binding (Rufer et al, 2015 ). Interestingly, tapasin interacts also with core TM9 of unassembled TAP1 which facilitates transporter stability and heterodimerization (Leonhardt et al, 2014 ).…”

Properties of the tapasin homologue TAPBPR

“…As a key component of the PLC, tapasin makes a number of essential contacts with the TAP transporters [1][2][3]32,33], ERp57 [34,35] and of course MHC class I (Figure 1). In contrast, our current, over-simplified model of TAPBPR is one in which it performs peptide-editing on MHC class I in isolation, without any additional co-factors (Figure 1).…”