Submitted December 23, 2006
Returned for revision January 26, 2007
Accepted February 20, 2007
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Vertebrate Yolk Complexes and the Functional Implications
of Phosvitins and Other Subdomains in Vitellogenins
Roderick Nigel Finn *
* To whom correspondence should be addressed. E-mail: nigel.finn{at}bio.uib.no.
Abstract
In non-placental or non-trophotenic vertebrates early
development depends on the maternal provision of egg yolk
that is mainly derived from large multi-domain
vitellogenin (Vtg) precursors. To reveal the molecular
nature of protein pools found in vertebrate oocytes,
published data for the N-termini of yolk proteins are
mapped to the deduced primary structures of their parent
Vtgs. Available evidence shows that the primary cleavage
sites of Vtgs are conserved, but the cleavage products
exist as multi-domain variants in the yolk protein pool.
The serine-rich phosvitin (Pv) domains are linearly
related to the Mr of the lipovitellin heavy chain. The
3-dimensional location of Pv is mapped to the outer edges
of the Vtg monomer where it is proposed to form
amphipathic structures that loop up over the lipid pocket.
In this locus it is proposed that Pv stabilises nascent
Vtg while it receives its lipid cargo, and thus
facilitates the hepatic loading and locking of lipid
within the Vtg (A-sheet)-(N-sheet)-(LvL) cavity, and
enhances its solubility following secretion to the
circulating plasma. The C-terminal regions of Vtgs are
homologous to human von Willebrand factor type D domains
(Vwfd), a conserved cysteine rich molecule with homologous
regions prevalent in Vtgs, lipophorins, mucins, integrins,
and zonadhesins. Unlike human Vwfd, lower vertebrate Vwfds
do not contain "RGD" motifs associated with extracellular
matrix binding. Its function in Vtg is unknown, but the
lubricant properties associated with mucins, and the cell
adhesion properties associated with integrins and
zonadhesins implicate Vwfd in the origin of hemostatic
platelet aggregation. Similarly, the proteolytic
inhibitory properties associated with the binding of
factor VIII in humans, suggests that Vwfd may stabilize
Vtg during passage in the systemic circulation.
Key words:
Gamete Biology •
Developmental biology •
Gametogenesis •
Oocyte development
