Sec-independent protein insertion into the inner E. coli membrane. A phenomenon in search of an explanation

Translocation of proteins through the inner membrane of E. coli is normally catalyzed by the so-called sec-machinery. Yet, many integral inner membrane proteins appear not to require a fully functional sec-machinery for proper insertion, in spite of the fact that sometimes quite sizable domains have to be translocated to the periplasmic side. This review will focus on recent studies of sec-independent translocation events in an attempt to pin-point the main differences between sec-dependent and sec-independent translocation.