Summary⌽X174 lysis protein E-mediated lysis of Escherichia coli is characterized by a protein E-specific fusion of the inner and outer membrane and formation of a transmembrane tunnel structure. In order to understand the fusion process, the topology of protein E within the envelope complex of E. coli was investigated. Proteinase K protection studies showed that, during the time course of protein E-mediated lysis process, more of the fusion protein E-FXa-streptavidin gradually became accessible to the protease at the cell surface. These observations postulate a conformational change in protein E during induction of the lysis process by movement of the C-terminal end of the protein throughout the envelope complex from the inner side to the outer side spanning the entire pore and fusing the inner and outer membranes at distinct areas. The initiation mechanism for such a conformational change could be the cis-trans isomerization of proline residues within ␣-helical membrane-spanning segments. Conversion of proline 21, presumed to be in the membrane-embedded ␣-helix of protein E, to alanine, glycine, serine and valine, respectively, resulted in lysis-negative E mutant proteins. Proteinase K accessibility studies using streptavidin as a reporter fused to the P21G mutant protein showed that the C-terminal part of the fusion protein is not translocated to the outer side of the membrane, suggesting that this proline residue is essential for the correct folding of protein E within the cell wall complex of E. coli. Oligomerization of protein P21G-StrpA was not disturbed.

As a tool for determining the topology of the small, 91-amino acid phi X174 lysis protein E within the envelope complex of Escherichia coli, a lysis active fusion of protein E with streptavidin (E-FXa-StrpA) was used. The E-FXa-StrpA fusion protein was visualised using immune electron microscopy with gold-conjugated anti-streptavidin antibodies within the envelope complex in different orientations. At the distinct areas of lysis characteristic for protein E, the C-terminal end of the fusion protein was detected at the surface of the outer membrane, whereas at other areas the C-terminal portion of the protein was located at the cytoplasmic side of the inner membrane. These results suggest that a conformational change of protein E is necessary to induce the lysis process, an assumption supported by proteinase K protection studies. The immune electron microscopic data and the proteinase K accessibility studies of the E-FXa-StrA fusion protein were used for the working model of the E-mediated lysis divided into three phases: phase 1 is characterised by integration of protein E into the inner membrane without a cytoplasmic status in a conformation with its C-terminal part facing the cytoplasmic side; phase 2 is characterised by a conformational change of the protein transferring the C-terminus across the inner membrane; phase 3 is characterised by a fusion of the inner and outer membranes and is associated with a transfer of the C-terminal domain of protein E towards the surface of the outer membrane of E. coli.

Petra Schön (Hrsg. im Auftrag des Landkreises Heilbronn): Mensch – Kultur – Heimat: Was Kleindenkmale aus dem Landkreis Heilbronn erzählen (Schriftenreihe des Landkreises Heilbronn, Band 6). Verlag regionalkultur, Ubstadt-Weiher, 2018. 295 Seiten mit 1217 Abbildungen. Gebunden € 19,90. ISBN 978-3-95505-050-4

Kleindenkmale im Landkreis Rottweil. Im Auftrag des Landkreises Rottweil hg. von Bernhard Rüth und Armin Braun. Ubstadt-Weiher: verlag regionalkultur 2018. 320 S. mit 532 farb. Abb. ISBN 978-3-89735-973-4. € 24,80

Mensch – Kultur – Heimat. Was Kleindenkmale aus dem Landkreis Heilbronn erzählen (Schriftenreihe des Landkreises Heilbronn, Bd. 6), hg. von Petra Schön, mit Beiträgen von Christian Himmelhan und Petra Schön, Ubstadt-Weiher/Heidelberg/Basel: verlag regionalkultur 2018. 296 S. mit 1217 farb. Abb. ISBN 978-3-95505-050-4. € 19,90