Part of the twin-arginine translocation (Tat) system that transports large folded proteins containing a characteristic twin-arginine motif in their signal peptide across membranes. Together with TatC, TatB is part of a receptor directly interacting with Tat signal peptides. TatB may form an oligomeric binding site that transiently accommodates folded Tat precursor proteins before their translocation. The Tat system comprises two distinct complexes: a TatABC complex, containing multiple copies of TatA, TatB and TatC subunits, and a separate TatA complex, containing only TatA subunits. Substrates initially bind to the TatABC complex, which probably triggers association of the separate TatA complex to form the active translocon. Essential for growth. Belongs to the TatB family.