Ten years ago when Nathan Coussens, a young researcher at Iowa noticed shiny crystals spilling out of a roach’s gut, he was shocked an dhis fellow researcher dismissed them as organic salt crystals commonly found in the gut of insects, birds and reptiles. But Coussens didn’t stop there and his persistence proved that they were indeed milk protein.
The roach species called Diploptera punctata in which Coussens first discovered these crystals, is a strange insect with a mammal-like quality – it “lactates”, producing milk to feed its brood. The crystals that Coussens had found were actually milk proteins that had crystallised within the guts of roach younglings.
Now, the same mystery has been soled by Bangalore-based Institute for Stem Cell Biology and Regenerative Medicine (inStem). Ramaswamy S., then a faculty member at the University of Iowa (with Coussens who was his former student) and now a faculty member at inStem, admits, “I just thought they were uric acid crystals. But Nathan was right to have been so persistent. They were protein crystals.”
Although protein crystals obtained from living systems are not unknown, these are generally formed within cells, tending to be small and limited in size by the volume of the cell they grow in. Such crystals are not very useful for studies on protein structure. In contrast, the milk crystals within roach guts grow large enough to be used for X-ray crystallography, a technique employed to elucidate protein structure.
The large protein crystals from roach guts were in themselves peculiar – a structural biologist’s fantasy given life – but solving their structure led to results that were curiouser and curiouser.
Usually, scientists try very hard to obtain pure proteins with which to make crystals for X-ray crystallography studies as X-ray crystallography generally requires crystals with well-ordered structures. “If there is heterogeneity in protein crystals, there is so much random disorder that they don’t diffract at all,” says Sanchari Banerjee, one of the main authors of the paper.
The milk protein crystals are like a complete food – they have proteins, lipids and sugars. “If you look into the protein sequences, they have all the essential amino acids,” said Banerjee.
Now, armed with the gene sequences for these milk proteins, Ramaswamy and colleagues plan to use a yeast system to produce these crystals en masse. “They’re very stable. They can be a fantastic protein supplement,” says Ramaswamy.
So, we can expect roach milk on shelves soon to supplement our protein requirments.
The work described here has been published as a paper titled “Structure of a heterogeneous, glycosylated, lipid-bound, in vivo-grown protein crystal at atomic resolution from the viviparous cockroach Diploptera punctata” in the journal IUCrJ in July 2016.