Functional role of C-terminal sequence elements in the transporter associated with antigen processing

Area of Science:

• Molecular Biology
• Immunology
• Protein Structure and Function

Background:

• The transporter associated with antigen processing (TAP) complex is essential for delivering antigenic peptides into the endoplasmic reticulum (ER) for MHC class I binding.
• TAP comprises two subunits, TAP1 and TAP2, each containing a transmembrane domain (TMD) and a nucleotide-binding domain (NBD).
• The distinct biochemical properties of the two TAP-NBDs regulate different steps in peptide translocation.

Purpose of the Study:

• To identify the specific sequence elements responsible for the asymmetrical function of the TAP-NBDs.
• To elucidate the molecular mechanisms underlying the functional separation of TAP-NBD1 and TAP-NBD2.

Main Methods:

• Construction of chimeric rat TAP variants by systematically exchanging sequence regions between TAP-NBD1 and TAP-NBD2.
• Fine-mapping studies to pinpoint critical sequence elements responsible for NBD functional asymmetry.

Main Results:

• A nonhomologous region, encompassing the alpha6/beta10-loop and the downstream switch region, was identified as directly responsible for the functional separation of TAP-NBDs.
• The alpha6/beta10-loop dictates the distinct nucleotide binding of NBD1 and NBD2.
• The switch region plays a critical role in regulating functional cross-talk between TAP's structural domains.

Conclusions:

• The alpha6/beta10-loop and switch region form a minimal functional unit controlling the asymmetry of TAP-NBDs.
• These findings provide insights into the precise molecular determinants of TAP subunit functional divergence.
• Understanding TAP asymmetry is crucial for comprehending antigen presentation pathways.