Involved in the association of MHC class I with transporter associated with antigen processing (TAP) and in the assembly of MHC class I with peptide (peptide loading). Interacts with TAP1 and is thus a subunit of the TAP complex. Interaction with TAP1 is TAP2 independent and is required for efficient peptide-TAP interaction. Obligatory mediator for the interaction between newly assembled MHC class I molecules, calreticulin, ERp57 and TAP. Up to 4 MHC class I/tapasin complexes bind to 1 TAP (By similarity). The N-terminus is required for efficient association with MHC class I molecule and for a stable interaction between MHC I and calreticulin. Binding to TAP is mediated by the C-terminal region (By similarity).