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Structure of Lili Mip
Twenty-five distinct Lili-Mip complementary DNAs have been cloned and partially characterized that encode twenty-two Milk proteins. These peptides are each 171 amino acids long1.
Lili-Mip has been characterized as part of the lipocalin family consisting of one C-terminal alpha-helix and nine Beta strands that form a barrel to coordinate the lipid. The lipocalin family typically consists of lipophilic ligands in a cavity shaped by a common fold composed of a central Beta barrel.2 The significant differences between lipocalin-like proteins is the hydrophobic pocket used for lipid coordination and categorizes the type of ligand that can be accommodated. Lipocalin proteins are known to carry fatty acids and other lipophilic materials.4
Electron density maps of the structure revealed densities for glycosylation at Asn35, Asn66, Asn79 and Asn145. N-acetyl-D-glucosamine were found to be unique ligands associated with Lili-Mip. One Beta-mannose (BMA) and two NAGS were identified at positions 35 and 79. Several other NAGs were identified at Asn145 (two) and one at Asn66.2 Models also coordinate lipids linoleic acid or oleic acid inside the hydrophobic pocket of Lili Mip. The pocket in the Lili-Mip structure is 15 Armstrong’s deep and can lodge up to 18-carbon fatty acid chain ligands. Aromatic residues (Phe76, Tyr84, Tyr88 and Phe100), combined with Leu113 and Glu38, delimit the deepest depression, notably through the formation of a stable π-stacking of Tyr88 and Phe100 rings that restrict the length of the lipid.2
Milk proteins are rich in leucine, valine, asparagine, lysine, proline, and tyrosine residues, each accounting for 7-8% of the amino acids. However, they are lower in methionine and tryptophan residues being only about 0.5% each.1 Lipids make up 16-22% of the dry weight, with cholesterol being the only steroid and linoleic acid the most abundant fatty acid. Fatty-acid chains in the barrel of Lili-Mip is presumed to be either linoleic acid or oleic acid.7
Function
Lili-Mip is a complete nutritious material as it provides all of the essential amino acids, carbohydrates from the attached glycans, and lipids. Lili-Mips are the major nutrient source for growth for embryos.
While measuring nutritional value based on the presence of essential amino acids, there are eight conserved regions in Lili-Mip which suggest the Milk proteins might have a function in addition to nutrition.1
The abundance of linoleic acid in the hydrophobic pocket of milk protein Lili-Mip of pregnant D. punctata is significant as linoleic acid is essential to most insects, and in other animals, lipocalins may transport hydrophobic molecules such as cholesterol and linoleic acid that cannot be synthesized by insects.2
The relationship between lipocalin family of proteins and Lili Mip may suggest that Lili Mip may function in the transport of cholesterol to the embryos as lipocalin proteins have been documented to bind a variety of hydrophobic molecules as well as cholesterol.2
Heterogeneity in Lili-Mip amino-acid sequence
Heterogeneity is observed in the primary protein structure evident by glycosylation and lipid content of Lili-Mip; however, the function of heterogeneity is currently unclear.2 Lili-Mip crystals containing multiple proteins with differing primary amino-acid sequences have been identified, heterogeneity for six of the 28 residues could be visualized clearly in the electron density maps.2
The significance of heterogeneity here is that there is no identified or reported crystal structure that has heterogeneity in protein sequence, carbohydrate, and lipid content together.2 There are some examples of heterogeneity in protein sequence but with glycosylation and ligand binding independently. Also notable is the ability for Lili-Mip’s ability to diffract to atomic resolution.2 The molecular weight ranges from 27 to 37 kDa which can be partially explained by the differential glycosylation of Milk proteins. The Lili-Mip contain four to six potential glycosylation sites that might be differentially glycosylated depending on the protein folding. 1
Furthermore, all heterogeneous residues of the protein are located on the surface of and not inside the barrel. This suggests the binding pocket is conserved.2
Medical Implications
According to the UNICEF, nearly half of all deaths in children under the age of five are attributable to under nutrition.5 This organization and the World Health Organization’s data suggests this translates to about three-million deaths of children a year.5 Sources to treat this malnutrition are vital to the survival of millions of people.
Given the discovery of the nutritious capabilities of Lili-Mip has opened doors into research about reproducibility to treat malnutrition and other protein-deficiency diseases. While milking cockroaches may not be the most productive, the discovery of this Milk protein will pave the way for an efficient method to reproducing it in mass.
Environmental Implications
Dairy milk and other alternatives are currently under environmental scrutiny as the amount of greenhouse gases produced by livestock raised for their milk and meat is too significant to ignore.6 While cockroach Diploptera punctate is far from the planet’s only solution to alleviate the stress of the accumulation of greenhouse gasses, there is potential for further research of the Milk protein produced by D. punctate as a more sustainable nutritious substances for our diets.
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References
