Heterohexamerin found in silkworm hemolymph

Insect hemolymph (analogous to human blood) contains several ~80 kDa storage proteins (SP) that play a role during metamorphosis. The SP proteins are also known as hexamerins because of their quaternary structure. It has been previously assumed that SP's formed homohexamers, i.e. were always composed of six identical subunits. Dr. Agnieszka Pietrzyk isolated a number of proteins from the hemolymph of Bombyx mori (silkworm) larvae and was able to crystallize the proteins from the SP fraction. It came as a big surprise, when the crystal structure of her hexamerin clearly showed that the complex is formed from two different proteins. Ultimately Agnieszka identified in her complex three copies of SP2 (shown in shades of blue) and three copies of SP3 (shown in shades of red). Proper identification of the two proteins was a big challenge and Agnieszka was able to accomplish this thanks to the high quality of her electron density maps, which she had to scrupulously confront with the amino acid sequences of the SP proteins known from the B. mori genome sequencing project. The protein glycosylation sites are shown in space-filling representation.

To isolate the proteins from the hemolymph, Dr. Pietrzyk had to use a sophisticated combination of chromatography techniques. Her crystallographic work was assisted by Mass Spectrometry analyses.

Dr. Pietrzyk's results were published in Acta Crystallographica (Acta Cryst. D69, 2353-2364, 2013).

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Last update: April 26, 2014 (MJ)