Structure of glycolipoprotein filaments from honeybee royal jelly solved at ETH Zurich
A recent "Nature Communications" paper by scientists from the Institute of Molecular Biology and Biophysics and the newly founded Cryo-EM Knowledge Hub of ETH Zurich reveals the unique three-dimensional structure of the glycolipoprotein filaments that maintain the viscosity of honeybee royal jelly.
Royal Jelly (RJ) is a secretion of the honeybee (Apis mellifera) that serves as nutrition for larvae. Larvae of workers and queens both feed on RJ during the first 3 days of larval development. While workers are fed pollen after day three, queen larvae receive RJ as sole nutrition during the entire larval development. In contrast to the horizontally oriented cells harboring the worker larvae, the larger queen cells are oriented vertically, and the queen larvae are embedded in RJ attached to the ceiling of the queen cells. A prerequisite of stable RJ attachment to the ceiling of queen cells is the extraordinary viscosity of RJ, which originates from high concentrations of long (> 1 micrometer), supramolecular glycolipoprotein filaments in RJ.
Scientists from the Institute of Molecular Biology and Biophysics and the newly founded Cryo-EM Knowledge Hub of ETH Zurich now succeeded in solving the three-dimensional structure of these remarkable filaments at 3.5 Å resolution (see picture). The building blocks of the filaments proved to be hetero-octameric, H-shaped protein complexes composed of four copies of the major royal jelly glycoprotein MRJP1 on the outside and four copies of the minor, hydrophobic subunit apisimin in the center that interact with 8 molecules of 24-methylene-cholesterol and lipids. In the RJ filaments, the hetero-octameric complexes are stacked such that every hetero-octamer rotates by 64 degrees relative to the previous hetero-octamer.
Filament assembly is triggered by the acidic pH of RJ (pH 4). This leads to protonation of glutamate residues of MRJP1 located at the interfaces between neighboring hetero-octamers, preventing mutual electrostatic repulsion of hetero-octamers. In contrast to RJ, the pH in the gut of the larvae is neutral (pH 7) so that the glutamate residues become negatively charged. This causes filament dissociation to hetero-octamers and loss RJ viscosity in the gut, making the hetero-octamers accessible for degradation by digestive enzymes.
Link to the paper in external page Nature Communications.