Lateral diffusion of GFP-tagged H2L(d) 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 loa ding. 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 leas t several minutes. It also suggests that the TAP complex is very large, per haps containing hundreds of proteins.