Dephosphorylation of chicken riboflavin-binding protein and phosvitin decreases their uptake by oocytes.

Riboflavin-binding protein (RBP) and phosvitin are phosphoglycoproteins transferred from the plasma of laying hens into the yolk of developing oocytes. We have examined the effect of phosphate removal on this yolk deposition process. Unmodified yolk RBP and phosvitin contain, respectively, 8.3 and 109 residues of phosphate/molecule. Complete dephosphorylation of yolk RBP caused a 20-min decrease in the plasma clearance half-life an 87% decrease in the uptake of the protein into oocytes in vivo. Although partially desialylated, dephospho-yolk RBP was identical with the native protein by several criteria, including riboflavin-binding capacity, mobility on SDS-polyacrylamide gels, and circular dichroism. A series of partially dephosphorylated yolk RBP samples, prepared by limited enzymatic hydrolysis, was indistinguishable from native yolk RBP by all criteria except phosphate content. Removal of the 1st phosphate residue decreased uptake of yolk RBP into oocytes by about 60%. Uptake into oocytes could not be restored to dephospho-yolk RBP by addition of anionic groups by succinylation. However, succinylation of native yolk RBP decreased its deposition into oocytes to the same extent as dephosphorylation. Partial dephosphorylation of phosvitin also had marked effects. The plasma clearance of dephosphophosvitin (70% of phosphate removed) was much faster than native phosvitin. After 4 h, 15% of injected 125I-phosvitin remained in circulation compared with only 3.8% of 125I-dephosphophosvitin. The uptake of dephosphovosvitin into oocytes was 79% less than that of native phosvitin. In vitro, 125I-phosvitin bound specifically to a preparation of oocyte plasma membranes as indicated by competition with unlabeled phosvitin but not with RBP. The specific binding of dephosphophosvitin was 96% less than that of native phosvitin and it could be displaced equally well by phosvitin or yolk RBP.