FIGURE 2.

Topography of protease-accessible sites. Membrane-resident GLUT1 was digested with trypsin or α-chymotrypsin and then analyzed by reverse phase HPLC-ESI-MS/MS. Peptides containing the indicated cleavage sites were positively identified by MS/MS. A, GLUT1 contains 35 potential trypsin cleavage sites (16 lysine residues and 19 arginine residues). Thirty-two are cleaved by trypsin. , observed lysine cleavage sites; •, lysine residues not observed as cleavage sites; ⋄, observed arginine cleavage sites; ♦, arginine residues not observed as cleavage sites. Arg⁴⁰⁰ (gray ♦) is flanked by N- and C-terminal proline residues and is not a potential trypsin cleavage site. B, GLUT1 contains 197 potential α-chymotrypsin cleavage sites (Phe, Tyr, Trp, Leu, Met, Ala, and Glu). The 52 detected cleavage sites are indicated by . Potential α-chymotrypsin (α-CT) cleavage sites are present in all TM domains. TMs are colored as in Fig. 1.