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The structure and the nutritive power of cockroach milk crystals

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A research work on generation of natural crystals of proteins by a cockroach viviparous species has been partly investigated on the PROXIMA-1 beamline.

It all began with curiosity. When Nathan Coussens, a young researcher in Iowa noticed shiny crystals spilling out of a roach’s gut, he was intrigued.  It was around 10 years ago. The subject has been making his way since, and it is through an international collaborative effort involving Americans, Indians, Canadians, Japanese and French (PROXIMA-1) teams that the mystery surrounding those crystals has been solved.

The crystals come from the nutritive substance secreted by females for their offspring’s growth. This “milk” naturally turns into crystals in the infants midguts, then constituting a stock available regarding the younglings’ nutritive needs.

In order to understand at the atomic scale the composition of those crystals which contain proteins, lipids and sugars (which means it is a complete nutrient), researchers determined their 3D structures using X-ray diffraction on the PROXIMA-1 beamline. Despite this complex and heterogeneous composition of the mixture, crystals formed in vivo are stable and diffract remarkably well.

Figure: 3D representation of the cockroach milk protein ‘Lili-Mip’; in the center, a lipid (yellow) is surrounded by a chain of amino-acids on which several sugar molecules (orange) are fixed. This structure has been determined at very high resolution thanks to the analysis (x-ray diffraction) of crystals naturally produced by the females of the viviparous Diploptera punctata cockroach.

They are very energetic (three times more than the same quantity of cow milk) and X-rays reveal that the extended release of their components come from their barrel-like shape.

The subject, which began with curiosity, today becomes a full part research project at SOLEIL. In this program, the PROXIMA-1 team is especially interested in the study of natural crystallization modes. The structure resolution of those in vivo crystals synthetized by cockroaches is just a first step and opens the way to many more crystals from living systems and much more exotic ones.