Type V Protein Secretion: Simplicity Gone Awry?

Since its discovery in the late 1980's, the family of secreted proteins termed the autotransporters has been expanding continuously to become the largest group of secreted proteins in Gram-negative bacteria. The type V secretion pathway, which includes the autotransporters (type V_a) together with the two-partner secretion system (type V_b) and the Oca family (type V_c), can be defined by secreted proteins that are (i) translocated across the outer membrane via a transmembrane pore formed by a ß-barrel and (ii) contain all the information required for translocation through the cell envelope. In the light of new discoveries and controversies in this research field, the secretion process of autotransporters, or the type V_a secretion system, will be discussed here and placed in the context of the more general field of bacterial protein translocation.