Submitted July 18, 2006
Returned for revision August 24, 2006
Accepted February 20, 2007
Gamete Biology
The Maturational Disassembly and Differential Proteolysis
of Paralogous Vitellogenins in a Marine Pelagophil
Teleost: A Conserved Mechanism of Oocyte Hydration
Roderick Nigel Finn *
* To whom correspondence should be addressed. E-mail: nigel.finn{at}bio.uib.no.
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 encoding
paralogous vitellogenins (cDNAs: hhvtgAa and
hhvtgAb; encoding proteins: HhvtgAa and HhvtgAb)
were cloned from non-estrogenized Atlantic halibut and the
N-termini of their sub-domain structures mapped to the
oocyte and egg yolk proteins (Yp). The maturational oocyte
Yp degradation products were further mapped to the free
amino acid (FAA) pool in the ovulated egg. Deduced amino
acid sequences conformed to the linear
NH2-(LvH-Pv-LvL-beta'-CT)-COO- structure for complete
teleost Vtgs. However, the Yps did not match the expected
cleavage products of complete Vtgs. Specifically, the
phosvitin sub-domain of the HhvtgAa paralogue remains
covalently attached to the lipovitellin light chain, while
the phosvitin sub-domain 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
lamprey and other teleosts. A small part of the C-teminal
end of the LvH-Ab is proteolysed to FAA, together with the
phosvitin, beta'-component, and much (~65%) of the
lipovitellin light chain (LvL-Ab). The independently
measured FAA pool in the ovulated egg corroborates that
calculated from the differential proteolysis of the Yps.
Due to 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.
Key words:
Gamete Biology •
Gametogenesis •
Meiosis •
Oocyte development •
Ovulation
