Reversible conversion between planar nanosheets and 3D structures in response to specific triggers would enable construction of nanodevices with useful electronic, optical, mechanical, biological, and surface properties. Here, we describe controlled formation, stabilization, and 2D-3D conversion of noncanonical planer lipid nanosheets. The lipid nanosheets were generated from vesicles by the addition of mixture of amphiphilic peptides and cationic copolymers having peptide-chaperoning activity. By refining the copolymer structure, autonomous and reversible conversion between 2D nanosheets and cell-sized 3D vesicles in response to triggers, such as pH and enzymatic activity, was achieved.We have reported that a cationic graft copolymer with a polycation backbone and water soluble graft chains has chaperone-like activity toward DNA [6] and peptides. [7] Poly(allylamine)-graft-dextran (PAA-g-Dex, Figure 1a) activates membrane-disruptive acidic peptide, E5, by facilitating folding of the peptide into an active amphiphilic helical conformation ( Figure 1b). E5 chaperoned by the copolymer had considerably higher membrane-disruptive activity than that observed for E5 alone. [7] To understand the effect of the copolymer and E5 on lipid membrane, a suspension of fluorescent-labeled vesicles was incubated with E5, PAA-g-Dex having 92 wt% dextran, or their mixture under physiological salt and pH conditions (10 × 10 −3 m HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)-NaOH (pH 7.4), 140 × 10 −3 m NaCl). The addition of E5/PAA-g-Dex caused a change from circles to lines in confocal microscopic images ( Figure 1c,d and Figure S4, Supporting Information), suggesting that the spherical vesicle structure was changed to a sheet structure. 3D image reconstructed from z-stack images also suggested morphological change from vesicle to sheet by E5/PAA-g-Dex ( Figure S5, Supporting Information). From the images, we calculated fraction of nanosheet (number of nanosheets/number of all lipid assemblies × 100). Most vesicles (92%, n > 300) were converted to the sheet structure within 5 min of E5/PAA-g-Dex addition. Previously, mesoscale nanosheet structures were observed by the addition of proteins having FERM domain, [8] which are cytoskeletal proteins, to lipid vesicles. However, the efficiency of conversion to the nanosheet structure induced by the proteins Nanosheets have thicknesses on the order of nanometers and planar dimensions in the micrometer range. Nanomaterials that are capable of converting reversibly between 2D nanosheets and 3D structures in response to specific triggers can enable construction of nanodevices. Supra-molecular lipid nanosheets and their triggered conversions to 3D structures including vesicles and cups are reported. They are produced from lipid vesicles upon addition of amphiphilic peptides and cationic copolymers that act as peptide chaperones. By regulation of the chaperoning activity of the copolymer, 2D to 3D conversions are reversibly triggered, allowing tuning of lipid bilayer structures an...