Lateral diffusion of GFP-tagged H2Ld molecules and of GFP-TAP1 reports on the assembly and retention of those molecules in the endoplasmic reticulum

Lateral diffusion of GFP-tagged H2L(d) molecules in the ER membrane reports on their interaction with the TAP complex during synthesis and peptide loading. Peptide-loaded H2L(d) molecules diffuse rapidly, near the theoretical limit for proteins in a bilayer. However, these molecules are retained in the ER for some time after assembly. H2L(d) molecules, associated with the TAP complex, diffuse slowly, as does GFP-tagged TAP1. This implies that the association of H2L(d) molecules with the TAP complex is stable for at least several minutes. It also suggests that the TAP complex is very large, perhaps containing hundreds of proteins.