SecA helps proteins fold (right), unless they have a signal sequence (left).
Ehrmann/NAS
Secreted bacterial proteins are targeted to and pushed through the translocon by the protein SecA. Because proteins must be threaded through the translocon in an unfolded state, the authors guessed that SecA might prevent folding of signal sequence–containing, and therefore secreted, proteins. Instead, they found just the reverse—SecA promotes the folding of proteins that lack export signals.
SecA bound to unfolded proteins even if they did not contain signal sequences. For proteins lacking an export signal, SecA promoted their folding to an active state. Once folded, SecA no longer binds, so secretion is thwarted. SecA did not have chaperone activity...
