Fig. 1.

Unique N-terminal domains (TMD₀s) of TAP1 and TAP2 represent an autonomous membrane protein fold. a Structural model of the TAP complex [3, 11, 50]. CoreTAP is shown as a homology model based on the structure of Sav1866 [13, 15]. The extra N-terminal domain TMD₀ of each TAP subunit, illustrated as a four-helix bundle, recruits the tapasin-ERp57 conjugate via transmembrane domain interaction. b TMD^TAP1₀ and TMD^TAP2₀ are integral membrane domains. Membranes were prepared from transiently transfected HeLa cells and alkaline extraction was performed. I an aliquot (1/10) of the membranes before alkaline extraction to confirm expression, M and S membrane fraction and supernatant after treatment with carbonate buffer pH 11.5, respectively. Immunoblots were developed against TMD^TAP1/2₀, tapasin as integral type I membrane protein, and SRP54 as peripheral membrane protein