This entry represents the open β-sheet domain found in vitellinogen, which generally corresponds to a domain within the lipovitellin-1 peptide product. This domain adopts a structure consisting of several large open β-sheets [ PUBMED:12135361 ].

Vitellinogen precursors provide the major egg yolk proteins that … expand

This entry represents the open β-sheet domain found in vitellinogen, which generally corresponds to a domain within the lipovitellin-1 peptide product. This domain adopts a structure consisting of several large open β-sheets [ PUBMED:12135361 ].

Vitellinogen precursors provide the major egg yolk proteins that are a source of nutrients during early development of oviparous vertebrates and invertebrates. Vitellinogen precursors are multi-domain apolipoproteins 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 [ PUBMED:17314313 PUBMED:9692232 ].

In vertebrates, a complete vitellinogen is composed of an N-terminal signal peptide for export, followed by four regions that can be cleaved into yolk proteins: lipovitellin-1, phosvitin, lipovitellin-2, and a von Willebrand factor type D domain (YGP40). Vitellinogens are post-translationally glycosylated and phosphorylated in the endoplasmic reticulum and Golgi complex of hepatocytes, before being secreted into the circulatory system to be taken up by oocytes. In the ovary, vitellinogens bind to specific Vtgr receptors on oocyte membranes to become internalised by endocytosis, where they are cleaved into yolk proteins by cathepsin D. YGP40 is released into the yolk plasma before or during compartmentation of lipovitellin-phosvitin complex into the yolk granule.

The different yolk proteins have distinct roles. Phosvitins are important in sequestering calcium, iron and other cations for the developing embryo. Phosvitins are one of the most phosphorylated (10%) proteins in nature, the high concentration of phosphate groups providing efficient metal-binding sites in clusters [ PUBMED:17189915 PUBMED:8838584 ]. Lipovitellins are involved in lipid and metal storage, and contain a heterogeneous mixture of about 16% (w/w) noncovalently bound lipid, most being phospholipid. Lipovitellin-1 contains two chains, LV1N and LV1C [ PUBMED:12135361 PUBMED:9687371 ].