“…Chimeric substrates have also been widely analyzed in particular for biotechnological or vaccinal purposes [99,[131][132][133][134]. These studies revealed (i) the importance of the RTX motifs for secretion, although this varies widely among the secretion apparatus and chimeric proteins tested [24,28,91,135]; (ii) a prerequisite for efficient secretion of the protein substrate is that it can unfold, likely to pass through the TolC channel, about 30 Å wide [115,116]; and (iii) premature folding of the protein substrates or stabilization of their 3D fold (e.g., via cytoplasmic disulfide bond formation) prevents secretion by T1SS [99,101,133,136]. Chaperones, like SecB or GroEL are not needed for HlyA or PtrB secretion [19,100], and it has been proposed that the RTX motifs could act as an internal chaperone that can prevent the folding of the protein substrate [17].…”