Selective protein transport: Characterization and solubilization of the phosvitin receptor from chicken oocytes

Phosvitin (PV), a subunit of a female-specific protein, vitellogenin, binds to oocyte membranes with a KD of 10^?6 M. Binding reaches equilibrium within 30 min after incubation at 25°C. Bound ¹²⁵I-PV dissociates from the membrane with a t1/2 of 13 h when incubated in buffer. However, when ¹²⁵I-PV-labeled membranes are incubated in buffer containing 10^?5 M unlabeled PV, 50% of the initially bound ¹²⁵I-PV dissociates from the membrane within 10 min. These results support the conclusion that PV binds to a membrane-associated receptor. Solubilization studies show that Triton X-100 solubilizes up to 45% of the total membrane-bound ¹²⁵I-PV. Gel-exculsion chromatography of the solubilized material yields a 500,000 dalton ¹²⁵I-PV-containing complex separated from free ¹²⁵I-PV. The 500,000 dalton complex completely dissociates to yield free ¹²⁵I-PV when incubated with excess unlabeled PV. However, when incubated with (1) no addition, (2) IgG, or (3) serum albumin, the extent of dissociation is significantly reduced and is consistent with that which would be predicted on the basis of the observed dissociation rate in the absence of unlabeled PV. These results suggest that bound ¹²⁵I-PV can only be displaced by unlabeled PV. These results also indicate that the 500,000 dalton species is a solubilized PV-receptor complex and that it is possible to solubilize the PV-receptor in an active form.