Periodically curved bilayer structures observed in hyphal cells or stable L-form cells of a Streptomyces strain, and in liposomes formed by the extracted lipids

Periodically curved bilayer structures showing a tetragonal pattern were revealed by freeze-fracture electron microscopy in hyphal cells, stable L-form cells, and liposomes prepared from extracted lipids of Streptomyces hygroscopicus NG 33–354. The pattern is formed by alternating convex and concave curvatures of the bilayer. It has been found with different repeat distances (multiples of about 15 nm) and with a different degree of expression (from just visible to very pronounced). An interpretation as infinite periodic minimal surface (IPMS) structures is more probable than an inducement of the pattern by underlying small vesicles. The occurrence of nonbilayer textures and the similarity of the tetragonal pattern with a ‘bilayer sector’ from a cubic phase structure (Andersson, S. et al. (1988) Chem. Rev. 88, 221–242) support such an interpretation.