Effect of nonbilayer lipids on membrane binding and insertion of the catalytic domain of leader peptidase

Biological membranes contain a substantial amount of "nonbilayer lipids", w hich have a tendency to form nonlamellar phases. In this study the hypothes is was tested that the presence of nonbilayer lipids in a membrane, due to their overall small headgroup, results in a lower packing density in the he adgroup region, which might facilitate the interfacial insertion of protein s. Using the catalytic domain of leader peptidase (Delta2-75) from Escheric hia coli as a model protein, we studied the lipid class dependence of its i nsertion and binding. In both lipid monolayers and vesicles, the membrane b inding of (catalytically active) Delta2-75 was much higher for the nonbilay er lipid DOPE compared to the bilayer lipid DOPC. For the nonbilayer lipids DOG and MGDG a similar effect was observed as for DOPE, strongly suggestin g that no specific interactions are involved but that the small headgroups create hydrophobic interfacial insertion sites. On the basis of the results of the monolayer experiments, calculations were per-formed to estimate the space between the lipid headgroups accessible to the protein. We estimate a maximal size of the insertion sites of 15 +/- 7 Angstrom (2)/lipid molecu le for DOPE, relative to DOPC. The size of the insertion sites decreases wi th an increase in headgroup size. These results show that nonbilayer lipids stimulate the membrane insertion of Delta2-75 and support the idea that su ch lipids create insertion sites by reducing the packing density at the mem brane-water interface. It is suggested that PE in the bacterial membrane fa cilitates membrane insertion of the catalytic domain of leader peptidase, a llowing the protein to reach the cleavage site in preproteins.