Membrane localization of transporter associated with antigen processing (TAP)-like (ABCB9) visualized in vivo with a fluorescence protein-fusion technique.

Transporter associated with antigen processing (TAP)-like (TAPL, ABCB9) is a half-type ATP binding cassette (ABC) protein belonging to subfamily B highly homologous to the TAP, a hetero-dimeric complex consisting of a TAP1 and a TAP2 subunit. Human TAPL, to which was tagged with green fluorescence protein (GFP) at its carboxyl terminus (TAPL-GFP), showed fluorescence on intracellular membranes similar to TAP1-GFP. A truncated form of TAPL-L-GFP (M1-S275 was followed by GFP) showed a similar cellular fluorescence pattern to TAPL-GFP. However, the fluorescence of TAPL-S-GFP (M1-G75) was distributed over all the cellular membranes including plasma membrane, indicating that the amino terminal region of TAPL (M1-S275) is essential for its localization to the intracellular membranes. A co-expression study demonstrated that TAPL-S-GFP was co-localized with TAPL-DR (DsRed-tagged TAPL) or TAP1-DR, suggesting that TAPL is able to interact with not only itself but also with TAP1 through the M1-G75 region of TAPL. It is also proposed that a further downstream sequence of TAPL would confine the distribution of TAPL-S-GFP to the intracellular membranes. Similarly, the distribution of TAP2-S-GFP (M1-R88) was restricted to the intracellular membranes by TAPL-DR or TAP1-DR, indicating that the M1-R88 region of TAP2 is able to interact with TAPL as well as TAP1. Therefore, TAPL would form a homo-dimer with itself, and a hetero-dimer with TAP1 and TAP2. TAPL-GFP was co-localized with the fluorescence endoplasmic reticulum (ER) marker, suggesting that TAPL is mainly localized to the ER in the intracellular membranes.