Figure I. FP fusion protein design.

A. Illustration of a typical resident ER luminal protein primary sequence. The Signal Sequence (SS) occurs at the extreme NH₂ terminus and the KDEL ER retrieval motif occurs at the absolute COOH terminus. Placement of an FP at either the NH₂ terminus or COOH terminus will disrupt essential position-dependent targeting information and are strongly discouraged (red Xs). In contrast, placement of the FP after the SS or before the KDEL will generate a protein correctly targeted to and retained in the ER lumen. B. The resulting FP-fusion protein, when the FP is placed immediately before the KDEL sequence. Note that the NH2 and COOH termini of GFP occur at the same end of the β barrel structure.