It is generally assumed that preprotein substrates must be presented in an unfolded state to the bacterial Sec-translocase in order to be translocated. Here, we have examined the ability of the Sec-translocase to translocate folded preproteins. Tightly folded human cardiac Ig-like domain 12 7 fused to the C terminus of proOmpA is translocated efficiently by the Sectranslocase and the translocation kinetics are determined by the extent of folding of the titin 127 domain. Accumulation of specific translocation intermediates around the fusion point that undergo translocation progress upon ATP binding suggests that the motor protein SecA plays an important and decisive role in promoting unfolding of the titin 127 domain. It is concluded that the bacterial Sec-translocase is capable of actively unfolding preproteins. (C) 2007 Elsevier Ltd. All rights reserved.