The family of PulD proteins, which has been characterized in a wide variety of microorganisms, comprises several membrane-associated proteins essential for the transport of macromolecules across bacterial membranes. These proteins are involved in the transport of complex structures (such as phage particles, DNA) or various proteins (such as extracellular enzymes and pathogenicity determinants). Amino acid sequence analysis revealed a possible modular organisation of proteins of this superfamily, with highly conserved C-terminal domains and dissimilar N-terminal domains. In the C-terminal domain, four highly conserved regions have been found, one of them containing a remarkable common motif: (V, I)PXL(S, G)XIPXXGXLF. Structural comparisons between the N-terminal domains indicate that proteins of this superfamily can be divided into at least two subgroups, probably reflecting the existence of distinct secretion mechanisms. This implies that members of the superfamily of PulD-related proteins are independently involved in (1) the general secretory pathway, (2) a new signal-peptide-independent secretion pathway found in several bacterial pathogens, and possibly in (3) the translocation of bacteriophage particles through the bacterial cell envelope.