Structure of phosvitin in amphibians
Phosvitin is the principal phosphoprotein in the eggs of numerous vertebrates (reviewed in [15]). In Xenopus laevis, a species closely related to Rana pipiens, phosvitin is one of three chains in the protein complex called Vitellogenin-A2 (1807 amino acids; Accession #P18709). The Protein Structure Prediction Server (PSIPRED) [16] was used to examine the secondary structure of the serine-rich phosvitin sequence (ca. 1126 - 1321). As shown in Figure 7, the predicted structure was relatively complex, with 6 α-helixes. Going from the N- to C-terminal ends, the sequence contains blocks of 38, 8, 28, 13 and 13 serine residues. Serines are generally excluded from helix-containing regions. Thus, each phosvitin molecule contains at least 100 serine residues as possible phosphorylation-dephosphorylation sites. Based on our data, it appears that progesterone selectively stimulates phosphorylation of about one of every 5 serines.
Figure 7  Predicted structure of the frog phosvitin component of Vitellogenin-A2 (1807 amino acids; Accession #P18709). The Protein Structure Prediction Server (PSIPRED) [16] was used to examine the possible conformation of the serine-rich phosvitin sequence (ca. 1126 - 1321). Line 1 indicates the confidence of the prediction, line 2 the relative position of each helix, line 3 whether the residue is part of a helix, strand or coil (see legend), and line 4 indicates the amino acid. Going from N- to C-terminal end, the sequence contains blocks of 38, 8, 28, 13 and 13 serine residues.

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