Atomic structure of Lanreotide nanotubes revealed by cryo-EM

Abstract

Functional and versatile nano- and microassemblies formed by biological molecules are found at all levels of life, from cell organelles to full organisms. Understanding the chemical and physicochemical determinants guiding the formation of these assemblies is crucial not only to understand the biological processes they carry out but also to mimic nature. Among the synthetic peptides forming well-defined nanostructures, the octapeptide Lanreotide has been considered one of the best characterized, in terms of both the atomic structure and its self-assembly process. In the present work, we determined the atomic structure of Lanreotide nanotubes at 2.5-Å resolution by cryoelectron microscopy (cryo-EM). Surprisingly, the asymmetric unit in the nanotube contains eight copies of the peptide, forming two tetramers. There are thus eight different environments for the peptide, and eight different conformations in the nanotube. The structure built from the cryo-EM map is strikingly different from the molecular model, largely based on X-ray fiber diffraction, proposed 20 y ago. Comparison of the nanotube with a crystal structure at 0.83-Å resolution of a Lanreotide derivative highlights the polymorphism for this peptide family. This work shows once again that higher-order assemblies formed by even well-characterized small peptides are very difficult to predict.

Polymorphism in peptide self-assembly visualized

“…This finding of conformational polymorphism by Pieri et al. ( 5 ) can certainly have important influences on the properties of peptide assemblies discussed in preceding paragraphs.…”

“…The supramolecular assembly of peptides, often with engineered hydrophobicity, chirality, or other modifications, has proven useful in making materials ( 3 ), vaccines ( 4 ), and therapeutics, exemplified in the PNAS paper by Pieri et al. ( 5 ), in which the structure of the assembly formed from the synthetic octapeptide Lanreotide has been determined at high resolution by cryotransmission electron microscopy (cryo-TEM).…”

“…Lanreotide is an eight-residue peptide, whose sequence is H 2 N-NahCY D WKVCTNH 2 , where Nah is D-naphthylalanine, comprising a precise sequence of aromatic, aliphatic, hydrophobic and hydrophilic, and charged residues. An intramolecular disulfide bond constrains this peptide into a β-hairpin that then assembles in water into nanotubes that are hundreds of micrometers long ( 5 ). There is an intriguing assembly phenomenon newly observed in the peptide assemblies of Lanreotide ( 5 ), namely, conformational polymorphism.…”

“…An intramolecular disulfide bond constrains this peptide into a β-hairpin that then assembles in water into nanotubes that are hundreds of micrometers long ( 5 ). There is an intriguing assembly phenomenon newly observed in the peptide assemblies of Lanreotide ( 5 ), namely, conformational polymorphism. As just cited ( 10 ), supramolecular assembly has been shown to drive more ordered average peptide conformation, but assembly in a regular manner in the same nanotube of multiple, individually unique conformations of chemically identical peptides has not previously been identified.…”

“…A tool is demonstrated in PNAS by Pieri et al. ( 5 ), namely, cryoelectron microscopy, that is able reveal the structure of Lanreotide at 2.5-Å resolution. In this work, it is seen that the small Lanreotide peptide adapts to eight slightly different environments via conformational adjustments, which repeat in a highly regular manner over the length of the self-assembled nanotube.…”