X ray crystallography provides the information about 3D structure of a protein and enables to determine its function, but it can also be extremely helpful in amino acid sequence identification of unknown proteins, for instance, as it was in the case of several major hemolymph proteins of mulberry silkworm Bombyx mori. Among the mentioned hemolymph proteins two major groups can be distinguished: high molecular weight storage proteins (SPs), which are hexamers (thus also referred to as “hexamerins”) and 30-kDa lipoproteins (LPs). As the presented studies aimed at the structural analysis of the most abundant proteins in the hemolymph, the identities of the proteins isolated from its natural source were initially unknown. The level of homology among 30-kDa LPs is high and the percent of similar residues ranges from 50 to 95 %, the final identification of isolated 30-kDa LPs was possible only due to precise analysis of electron density maps.
However, the studies on high molecular weight SPs provided even more interesting results. At the beginning the protein isolated from high molecular weight fraction of hemolymph was identified as a pure SP2 according to the results of LC/MS/MS analysis. Later on, electron density maps revealed that the investigated structure represented a complex of two different SPs, SP2 and SP3. The hexameric complex was formed by three SP2 and three SP3 molecules. This was an important discovery, because both proteins were previously always described as homohexamers and according to the crystal structure they were described as a heterohexamer for the first time.
This research was supported in part by the European Union within the European Regional Development Fund and by grant 2011/03/B/NZ1/01238 from the National Science Centre.