The Maturational Disassembly and Differential Proteolysis of Paralogous Vitellogenins in a Marine Pelagophil Teleost: A Conserved Mechanism of Oocyte Hydration1

Abstract

A structural analysis of the differential proteolysis of vitellogenin (Vtg)-derived yolk proteins in the maturing oocytes of a marine teleost that spawns very large pelagic eggs is presented. Two full-length hepatic cDNAs (hhvtgAa and hhvtgAb) encoding paralogous vitellogenins (HhvtgAa and HhvtgAb) were cloned from nonestrogenized Atlantic halibut, and the N-termini of their subdomain structures were mapped to the oocyte and egg yolk proteins (Yps). The maturational oocyte Yp degradation products were further mapped to the free amino acid (FAA) pool in the ovulated egg. The deduced amino acid sequences conformed to the linear NH(2)-(LvH-Pv-LvL-beta'-CT)-COO(-) structure of complete teleost Vtgs. However, the Yps did not match the expected cleavage products of complete Vtgs. Specifically, the phosvitin subdomain of the HhvtgAa paralogue remains covalently attached to the lipovitellin light chain, while the phosvitin subdomain of the HhvtgAb paralogue remains covalently attached to a C-terminal fragment of the lipovitellin heavy chain (LvH). During oocyte hydration, the LvH of the HhvtgAa paralogue is disassembled and extensively degraded to FAA. In the HhvtgAb paralogue, the LvH is nicked in the C-sheet in a manner similar to that seen in lamprey and other teleosts. A small part of the C-terminal end of the LvH-Ab undergoes proteolysis to FAA, together with the phosvitin, beta' component, and much ( approximately 65%) of the lipovitellin light chain (LvL-Ab). The independently measured FAA pool in the ovulated egg corroborates that calculated from differential proteolysis of the Yps. Based on the 3:1 (HhvtgAb:HhvtgAa) Yp expression ratio, each paralogue contributes approximately equal amounts of FAA to the organic osmolyte pool of the hydrating oocyte during maturation.

Vertebrate Yolk Complexes and the Functional Implications of Phosvitins and Other Subdomains in Vitellogenins1

“…In addition, the N-termini of the LvL domains from VtgABa1, VtgABa2, and VtgABb1 have been identified [49]. These N-termini precisely match the cleavage sites of LvL-Aa and LvL-Ab revealed in the recent study on Atlantic halibut [5] and the fully conserved Phe in the alignment of Finn and Kristoffersen [6]. Since the LvH domains have been independently estimated at 120 kDa [48], which matches the molecular mass predicted from the sequenced genes [47,50], the reported molecular mass of Pv should match the molecular masses of 23.2 kDa and 23.7 kDa calculated for VtgABa2 and VtgABb1, respectively (Fig.…”

“…Within the chicken Vtg genes, up to five Pv variants exist. Smaller phosvettes, which range in size from 13 kDa to 18 kDa, and 'full' Pvs, which range in size from 28 kDa to 50 kDa, have been reported [5,[55][56][57][58][59][60]. The predicted molecular masses of the chicken Pv proteins cleaved between the conserved KKIL site and the conserved HhvtgAa Phe 1131 putative CatD cleavage site should be 27.9 kDa, 25.9 kDa, and 11.8 kDa for VtgAB1, VtgAB2, and VtgAB3, respectively (Fig.…”

“…Vitellogenins (Vtg) are large multidomain apolipoproteins that are the precursors of the major egg yolk proteins (Yps) in vertebrates and in most invertebrates [1][2][3][4][5][6]. In vertebrates, synthesis of Vtg occurs cotranslationally, primarily in the liver, as a result of a co-ordinated female-specific endocrine cascade that involves the brain, ovary, liver, and systemic circulation [7].…”

“…Due to the multiplicity of the Vtg family [5,6,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45], it has proven challenging to establish the precursor-product relationships between Vtgs and Yps based on their physical, chemical or antigenic properties. Given the increased information on the deduced primary structures of the parent Vtgs together with the N-terminal peptide data on the derivative Yps, it is now possible to ascertain whether vertebrate Vtgs are indeed cleaved into their constituent Lv, Pv, and b 0 Yps.…”

Purification and characterization of a novel incomplete-type vitellogenin protein (VgC) in Sakhalin taimen (Hucho perryi)

“…This accumulation reflects disparate rates of deposition into growing oocytes of the yolk proteins derived from each form of parent Vg, as suggested by previous studies of marine or estuarine pelagophils (Matsubara et al, 1999;Reith et al, 2001;Hiramatsu et al, 2002c;Sawaguchi et al, 2006;Finn, 2007;Kolarevic et al, 2008;. The composition of yolk proteins derived from multiple Vg subtypes can be regulated, theoretically, by: 1) the ratios in circulating levels of multiple Vg subtypes, 2) the numbers and/or affinity of ovarian receptor(s) for Vg, and 3) both of these two factors.…”