Figure 1: Flowchart of the method developed to identify extracellular lysine residues.

Isolated cells were labeled with a membrane-impermeable, lysine specific labeling agent (sulfo-NHS-SS-biotin). The membrane fraction was purified, solubilized and digested with different proteolytic enzymes. The modified peptides were isolated on a neutravidin agarose resin, then eluted and sequenced by tandem mass spectrometry. Labeled positions were used as extracellular constraints in the CCTOP topology prediction algorithm.