Vitellogenin

Vitellogenin (VTG or less popularly known as VG) (from latin vitellus, yolk, and gener, to produce) is a precursor protein of egg yolk normally in the blood or hemolymph only of females that is used as a biomarker in vertebrates of exposure to environmental estrogens which stimulate elevated levels in males as well as females.^[1] "Vitellogenin" is a synonymous term for the gene and the expressed protein. The protein product is classified as a glycolipoprotein, having properties of a sugar, fat and protein. It belongs to a family of several lipid transport proteins.

Vitellogenin is an egg yolk precursor protein expressed in the females of nearly all oviparous species including fish, amphibians, reptiles, birds, most invertebrates, and monotremes.^[2] Vitellogenin is the precursor of the lipoproteins and phosphoproteins that make up most of the protein content of yolk. In the presence of estrogenic endocrine disruptive chemicals (EDCs), male fish can express the Vg gene in a dose dependent manner. Vg gene expression in male fish can be used as a molecular marker of exposure to estrogenic EDCs.

Function

Vitellinogen precursors provide the major egg yolk proteins that are a source of nutrients during early development of egg-laying (oviparous) vertebrates and invertebrates. Vitellogenin is the precursor of the lipoproteins and phosphoproteins that make up most of the protein content of yolk. More specifically, the N-terminal domain functions as a signal peptide which helps export. Vitellinogen precursors are multi-domain apolipoproteins (proteins that bind to lipids to form lipoproteins), that are cleaved into distinct yolk proteins. Different vitellinogen precursors exist, which are composed of variable combinations of yolk protein components; however, the cleavage sites are conserved.

Components of vitellinogen

In vertebrates, a complete vitellinogen is composed of:

• an N-terminal signal peptide for export,
• and four regions that can be cleaved into yolk proteins, these are:

• • lipovitellin-1,
  • phosvitin,
  • lipovitellin-2,
  • von Willebrand factor type D domain (YGP40).^[3][4]

N-terminal domain-Vitellogenin lipid transport domain

This particular domain represents a conserved region found in several lipid transport proteins, including vitellogenin, microsomal triglyceride transfer protein and apolipoprotein B-100.^[5]

Vesicle trafficking

This particular domain, the Vitellogenin lipid transport domain, is also found in the Microsomal triglyceride transfer protein (MTTP) and in Apolipoprotein B. It aids cell trafficking and export of cargo.

Microsomal triglyceride transfer protein (MTTP)

Microsomal triglyceride transfer protein (MTTP) is an endoplasmic reticulum lipid transfer protein involved in the biosynthesis and lipid loading of apolipoprotein B. MTTP is also involved in the late stage of CD1d trafficking in the lysosomal compartment, CD1d being the MHC I-like lipid antigen presenting molecule.^[6]

Apolipoprotein B

Apolipoprotein B can exist in two forms: B-100 and B-48. Apolipoprotein B-100 is present on several lipoproteins, including very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and low density lipoproteins (LDL), and can assemble VLDL particles in the liver.^[7] Apolipoprotein B-100 has been linked to the development of atherosclerosis.

Vitellogenin and honey bees

Honey bees deposit vitellogenin molecules in fat bodies in their abdomen and heads. The fat bodies apparently act as a food storage reservoir. The glycolipoprotein vitellogenin has additional functionality as it acts as an antioxidant to prolong Queen bee and forager lifespan as well as a hormone that affects future foraging behavior.^[8] The health of a honey bee colony is dependent upon the vitellogenin reserves of the nurse bees - the foragers having low levels of vitellogenin. As expendable laborers, the foragers are fed just enough protein to keep them working their risky task of collecting nectar and pollen. Vitellogenin levels are important during the nest stage and thus influence honey bee worker division of labor.

A nurse bee's vitellogenin titer that developed in the first four days after emergence, affects its subsequent age to begin foraging and whether it preferentially forages for nectar or pollen. If young workers are short on food their first days of life, they tend to begin foraging early and preferentially for nectar. If they are moderately fed, they forage at normal age preferentially for nectar. If they are abundantly fed, immediately after emergence, their vitellogenin titer is high and they begin foraging later in life, preferentially collecting pollen. Pollen is the only available protein source for honey bees.

Vitellogenin and juvenile hormone feedback loop

For the majority of the investigated insect species it has been documented that juvenile hormone stimulates the transcription of the vitellogenin genes and the consequent control of vitellogenin production (cf. Engelmann, 1983; Wyatt and Davey, 1996).^[9][10]

The vitellogenin expression is part of a regulatory feedback loop that enables vitellogenin and juvenile hormone to mutually suppress each other. Vitellogenin and juvenile hormone likely work antagonistically in the honey bee to regulate the honey bees development and behavior. Suppression of one leads to high titers of the other.^[11]

It is likely that the balance between vitellogenin and juvenile hormone levels is also involved in swarming behavior.^[12]

Juvenile hormone levels drop in honey bee colonies pre-swarming and it is expected that vitellogenin levels would therefore rise. One may surmise, that swarming bees would want to pack along as much vitellogenin as possible to extend their lifespan and to be able to quickly build a new nest.

References

This article incorporates text from the public domain Pfam and InterPro IPR001747

See also

• glycoprotein
• lipoprotein
• Science Daily: Vitellogenin Is a 'Freight Train' for Health and Lifespan