Figure 1. Surfaceome tagging allows surveillance of Basigin (Bsg)-mediated monocarboxylate transporter (MCT) trafficking.

(A) Within the Golgi apparatus, MCT proteins associate with Bsg, which is trafficked to the cell surface via its signal peptide. (B) Cartoon depiction of Bsg and homologous members within the superfamily, neuroplastin (Nptn) and embigin (Emb). Purple bands indicate transmembrane domains. Orange bands indicate signal peptides. Pink dashes indicate sites of putative N-linked glycosylation. (C) Scheme of surfaceome labeling. GGGYC peptide-tagged cholesterol incorporates into the cell surface, and APEX2-LPETG is conjugated via the engineered sortase, eSrtA. Upon addition of biotin-phenol (BP) and H₂O₂, the cell surface-tethered APEX2 generates short-lived (< 1 ms) and reactive biotin radicals that covalently tag proximal (<20 nm) proteins.