Structural and functional characterization of the antimicrobial peptide transporter SbmA

Transport proteins are vital for cellular function, and SbmA, a unique bacterial transporter, stands out due to its ability to import a wide range of antimicrobial peptides despite lacking ATP-hydrolyzing domains typical of ABC transporters. Originally identified in E. coli for importing microcin B17, SbmA exhibits structural similarity to type IV ABC transporters but functions independently of ATP. Instead, it appears to act passively, facilitating peptide diffusion along a concentration gradient. Structural studies, including cryo-EM, show SbmA contains a hydrophilic cavity that accommodates diverse substrates and undergoes conformational changes enabling alternating access from the periplasm and cytosol.
Evolutionary analysis indicates SbmA likely derived from full-length ABC transporters like YddA by losing its nucleotide-binding domains. Functional studies confirmed that key transmembrane regions, particularly TM6a/b, enable conformational flexibility essential for transport. Protonation of specific residues may influence these transitions, although the exact role of protons remains unclear.
A novel in vitro proteoliposome-based assay was developed to directly observe SbmA-mediated peptide transport, using a fluorescent reporter system. Results showed that neither pH gradient nor membrane potential significantly affected transport, supporting the hypothesis of passive diffusion. A V102G mutant revealed disrupted structural integrity, offering further insight into conformational dynamics.
Overall, SbmA represents an evolutionarily distinct, ATP-independent transporter capable of broad substrate uptake. Its mechanism likely involves intrinsic conformational cycling rather than substrate-induced transitions. The assay and structural insights provide a platform for future studies on transporter function, evolution, and potential biotechnological applications.