Abstract
The Escherichia coli TolQ protein is a 230-amino acid integral cytoplasmic membrane protein required for the import of group A colicins, for infection by the filamentous phage, and for maintenance of the integrity of the bacterial envelope. TolQ is a polytopic protein with three membrane-spanning regions. The first membrane-spanning region has a 19-residue periplasmic NH2-terminal tail, while the second and third membrane-spanning segments are separated by a short 17-amino acid periplasmic loop. To study the membrane assembly of TolQ, fusions of different membrane-spanning regions were examined for their ability to insert in the absence of functional SecA or the membrane potential. Fusions containing the first membrane-spanning region plus the adjacent cytoplasmic domain and a construct containing the "hairpin loop," formed by the second and third membrane-spanning regions, insert in the absence of functional SecA. The fusion containing the second and third membrane-spanning regions required the membrane potential for insertion while the first membrane-spanning region was able to insert even in the absence of a membrane potential. Taken together, these results suggest that insertion of intact TolQ is not dependent on the Sec system, but does require the membrane potential.
Highlights
Properly localizes to the cytoplasmic membrane and has no effect on cell growth
Aminoterminal sequence analysis of overexpressed TolQ purified from the inner membrane revealed the presence of the initiating formylmethionine, which suggests that TolQ is synthesized without a signal sequence and is rapidly inserted into the cytoplasmic membrane [7]
On the basis of these and other observations, it has been proposed that inner membrane proteins which form “hairpin loops,” that is two membrane-spanning segments separated by a periplasmic domain of less than 60 residues, insert Sec independently [15]
Summary
Properly localizes to the cytoplasmic membrane and has no effect on cell growth These characteristics raise the question whether the Sec system is required for TolQ membrane insertion. On the basis of these and other observations, it has been proposed that inner membrane proteins which form “hairpin loops,” that is two membrane-spanning segments separated by a periplasmic domain of less than 60 residues, insert Sec independently [15]. If the two membrane-spanning regions are separated by a larger periplasmic region, the Sec system appears to be required for translocation. The topology of the TolQ protein (Fig. 1) would suggest that both the first membrane-spanning region and the hairpin loop formed by the second and third membrane-spanning regions are inserted into the membrane independent of the Sec proteins. These results are discussed in relation to the membrane assembly of TolQ
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
