Vitellogenin diversity in tropical lizards (Anolis pulchellus): identification and partial characterization

Mass spectrometry-based sequencing and SRM-based quantitation of two novel vitellogenin isoforms in the leatherback sea turtle (Dermochelys coriacea)

No biomarker has yet been discovered to identify the reproductive status of the endangered leatherback sea turtle (Dermochelys coriacea). Although vitellogenin (VTG) could be used for this, its sequence is not known in D. coriacea and no quantitative assay has been carried out in this species to date. Using de novo sequencing-based proteomics, we unambiguously characterized sequences of two different VTG isoforms that we named Dc-VTG1 and Dc-VTG2. To our knowledge, this is the first clear evidence of different VTG isoforms and the structural characterization of derived yolk proteins in reptiles. This work illustrates how massive de novo sequencing can characterize novel sequences when working on "exotic" nonmodel species in which even nucleotide sequences are not available. We developed assays for absolute quantitation of these two isoforms using selected reaction monitoring (SRM) mass spectrometry, thus providing the first SRM assays developed specifically for a nonsequenced species. Plasma levels of Dc-VTG1 and Dc-VTG2 decreased as the nesting season proceeded, and were closely related to the increased levels of reproductive effort. The SRM assays developed here therefore provide an original and efficient approach for the reliable monitoring of reproduction cycles not only in D. coriacea, but potentially in other turtle species.

Induction of vitellogenesis by estradiol-17beta and development of enzyme-linked immunosorbant assays to quantify plasma vitellogenin levels in green turtles (Chelonia mydas)

Treatment of juvenile green turtles (Chelonia mydas) with estradiol-17beta resulted in the induction of a 200 kDa plasma protein, consistent with vitellogenin (Vtg). The N-terminal 15 amino acids of the anion exchange purified protein shared sequence homologies with vitellogenins of several vertebrate species. Rabbit antiserum raised against purified Vtg recognized the plasma protein as well as several yolk proteins. Monoclonal antibody (Mab) HL1248, produced by inoculating mice with turtle yolk granules, showed specificity for plasma Vtg as well as a set of yolk proteins 120, 82, 43 and 32 kDa in size. The N-terminal 22 amino acids of the 43 kDa yolk protein was similar to the lipovitellin I subunit of Vtg of several vertebrate species. The peptide mass map of the 82 kDa yolk protein shared enough ions with that of purified plasma Vtg to support the conclusion that this protein was derived from plasma Vtg. Taken together, these results validate the specificity of Mab HL1248 for Vtg. Using purified Vtg concentration standards, competition and antigen capture enzyme-linked immunosorbant assays (ELISAs) were shown to quantitatively detect Vtg in green turtle plasma. Pre-induced plasma of juvenile turtles had Vtg levels of 2-4 micrograms/ml whereas post-estradiol exposure samples had 38-40 mg/ml. The plasma Vtg concentration of a nesting female turtle was 4.6 mg/ml, approximately 20-fold higher than that of a non-nesting adult female. The antigen capture ELISA will be useful in population studies of this endangered species, to detect vitellogenesis in females that will nest in a given year and to detect inappropriate Vtg levels in turtles exposed to xenoestrogens.

Vitellogenin and yolk protein processing in Bothrops jararaca (Wied), a tropical venomous snake

The plasma vitellogenin of Bothrops jararaca is composed of two subunits. The larger subunit (160 kDa) is phosphate rich and carbohydrate poor, while the smaller (110 kDa) is highly glycosylated and less phosphorylated. As in other vertebrates, the vitellogenin of B. jararaca is synthesized in the liver under estrogen control. The newly synthesized vitellogenin molecule is a 270 kDa polypeptide. This polypeptide originates the two subunits of the plasma vitellogenin by proteolytic cleavage. In the eggs of B. jararaca six main yolk polypeptides have been detected (113, 107, 104, 72, 27.2 and 20.7 kDa). Using phosphoprotein staining we have shown that the 72 kDa polypeptide is the larger phosvitin so far described in a vertebrate egg yolk. The 107 kDa yolk polypeptide also seems to be phosphorylated, but to a lesser extent than the phosvitin. The 104 kDa vitellin originates from the larger vitellogenin subunit while the 113 kDa vitellin originates from the smaller vitellogenin subunit. Based on these results we propose a general scheme for vitellogenin and vitellin processing in B. jararaca.