Transmembrane effector substrates of type IV secretion systems: mechanisms of secretion and insertion into host cell membranes.

Intracellular Gram-negative pathogens employ either type IVA or type IVB secretion systems (T4SSs) to translocate effector proteins into host cells, where they modulate cellular processes to facilitate infection and promote intracellular survival. Roughly one-third of these effectors harbor hydrophobic transmembrane domains and are thus destined for integration into host cell membranes during infection. Many of these transmembrane domain-containing effectors (TMEs) localize to the membrane of the pathogen-containing vacuole, thereby contributing to its formation and remodeling. Despite the biological relevance of TMEs, the detailed molecular mechanisms governing their translocation via T4SSs and subsequent membrane integration in the host cell remain insufficiently understood. In this review, the biophysical characteristics of T4SS-secreted TMEs are systematically examined, including predictions of membrane topology and hydrophobicity. These analyses are then contextualized through comparison with recent structural analysis of both T4ASS and T4BSS machineries, as well as with mechanistic principles of eukaryotic membrane protein biogenesis. This integrative approach enables the conceptual reconstruction of the potential pathways by which TMEs are translocated through the T4SS and subsequently targeted and inserted into host membranes, offering new mechanistic insights into the poorly understood handling of bacterial TMEs from both the pathogen and host perspectives.