User:Marilyn Vazquez Zarazua/Sandbox 1
From Proteopedia
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Background
The milk protein Lili-Mip owes its origins to the viviparity of the Pacific beetle cockroach Diploptera punctate.1 This species gives birth to live young and utilizes a reproductive strategy, viviparity, in which a supply of nutrients is provided to growing embryos in gestation periods of their lives. Lili Mip analysis revealed that milk proteins contain all the essential amino acids which a vital characteristic for a nutrient is.
The time to reproductive maturity is significantly reduced because of viviparity. This enhancement of larval development provides a 60-fold whole body increase in protein during embryonic development. The viviparity of this cockroach species involves the rapid development of embryos that are capable of drinking and storing complex nutrients concentrated in crystalline form.1
While few study has been conducted on the Lili-Mip, the direct crystallographic phasing and structure determination from crystals naturally grown in vivo rather than in vitro from over expressed proteins is the first report of its kind.1
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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. 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
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