TraB, as described originally in the delta-proteobacterium, is a protein with a C-terminal OmpA-like domain, and is encoded in an operon with TraA. Together TraAB make it possible for bacterial cells with close enough kinship to exchange outer membrane lipoproteins, such that certain motility defects in mutant cells can be corrected through the exchange. This exchange most likely involves membrane fusion events, as large amounts of lipid are also exchanged, and it is restricted by a bi-directional kin recognition requirement. Among wild-type cells, these exchanges likely help coordinate various social behaviors.
Jiyao W et al.(2023). "The conserved domain database in 2023.",