The Microbial Anti-Inflammatory Molecule (MAM) is a key protein processed and exported to <i>Faecalibacterium duncaniae</i> envelope

MAM (Microbial-Anti-Inflammatory Molecule) is a key effector protein with anti-inflammatory properties in <i>Faecalibacterium duncaniae</i>, a critical human gut microbiota species. Despite its importance, MAM function and molecular features remain poorly understood. This study elucidates MAM's physiological importance by examining its cellular localization, secretion dynamics, and structural organization. Mass spectrometry and immunogold labeling confirmed MAM as the most abundant protein in the cell envelope and the second most abundant in the overall proteome, localizing it to the bacterial surface. Bioinformatic and <i>in silico</i> analyses suggest that MAM contains an N-terminal leader peptide with motifs recognized by a Peptidase-domain-Containing ABC Transporter (PCAT), enabling cargo transport to the cell envelope. After N-terminal excision, the cargo protein could be transported to the cell envelope via this PCAT, where it could assemble into a hexameric structure, as revealed by docking and AlphaFold3 modeling. Such results were supported by electron microscopy showing a lattice-like organization on the bacterial surface. This work introduces a novel discussion about the singular organization of the <i>F. duncaniae</i> cell envelope, having MAM as a key component for the bacteria, supporting the understanding of the unique biology of <i>F. duncaniae</i> and its potential as a next-generation probiotic or live biotherapeutic.