Identification and Characterization of Protease-Resistant SecA Fragments: SecA Has Two Membrane-Integral Forms

We have identified and characterized the protease-resistant SecA fragments (X. Chen, H. Xu, and P. C. Tai, J. Biol. Chem. 271:29698–29706, 1996) through immunodetection with region-specific antibodies, chemical extraction, and sequencing analysis. The 66-, 36-, and 27-kDa proteolytic fragments in the membranes all start at Met(1), whereas the 48-kDa fragment starts at Glu(361). The overlapping of the sequences of the 66- and 48-kDa fragments indicates that they are derived from different SecA molecules. These two fragments were generated differently in response to ATP hydrolysis and protein translocation. Furthermore, the presence of membrane is required for the generation of the 48-kDa fragment but not for that of the 66-kDa fragment. These data suggest that there are two different integral forms of SecA in the membrane: SecA(S) and SecA(M). The combination of these two forms of SecA has several membrane-interacting domains. Both forms of SecA are integrated in the membrane, since both the 48- and 66-kDa fragments could be derived from urea- or Na(2)CO(3)-washed membranes. Moreover, all fragments are resistant to extraction with a high concentration of salt or with heparin, but the membrane-specific 48-kDa SecA domain is more sensitive to Na(2)CO(3) or urea extraction. This suggests that this domain may interact with other membrane proteins in an aqueous microenvironment and therefore may form a part of the protein-conducting channel.