Distinctive Properties of Mla Proteins Differentiate Them From Classical ABC Transporter Components

In Gram-negative bacteria, the non-canonical ABC transporter, namely, maintenance of lipid asymmetry (Mla) system, ferries phospholipids (PLs) between the inner (IM) and outer (OM) membranes to preserve the PL asymmetry of the OM. The system utilizes three sub-cellular complexes-lipoprotein MlaA-OmpC/F (OM), MlaC (periplasmic), and MlaFEDB complex (IM). The structural studies on the Mla system have primarily been dedicated to its organization in IM and transport mechanisms. The characteristics of the individual components of the Mla system are lacking in the literature. In this study, individual components, namely MlaA, MlaB, MlaE, and MlaF were analyzed using computational tools. This has resulted in the identification of unique features and their characterization, including understanding the dynamicity of the C-terminal extension (CTE) of MlaA, which protrudes into the periplasm and the orientation of the protein, as well as binding patterns. Utilization of artificial intelligence has led to the understanding of the conformational landscape of MlaA and the validation of the macromolecular arrangement of Mla systems. Based on the results obtained, we were able to propose a fascinating mechanism of ligand transport, namely, bait-capture-pull. Our results reveal the poorly understood interfaces of the MlaB-MlaF complex. Furthermore, the results also suggest that MlaE possesses an EQ loop, which helps maintain a unique orientation. Overall, the findings of this study provide a new perspective on non-vesicular PL transport mediated by the enigmatic Mla system, thereby providing a holistic understanding.