Identification and localization of integrin subunits in oocytes and eggs of the mouse

Results of a recent study have implicated egg integrins in sperm binding to the egg plasma membrane (Blobel et al., 1991: Nature 356:248-252). In this report, immunoprecipitation was used to identify, and confocal immunofluorescence microscopy was used to localize, several different integrin subunits in mouse eggs. Antibodies to α₂, α₅, α(v), and β₁ subunits, as well as antibodies to the fibronectin receptor (FNR; α₅β₁ and/or α₃β₁) and vitronectin receptor (VNR; α(v)β₃ and/or α(v)β₅), detect polypeptides of the appropriate molecular weights following immunoprecipitation. β₁ is localized preferentially to either the microvillar or amicrovillar membrane/cortical regions of eggs, and these asymmetric localizations depend on the antibody used. Proteins recognized by anti-FNR antibodies are localized preferentially to the amicrovillar membrane/cortical region. Germinal vesicle-intact oocytes display a symmetric plasma membrane distribution using β₁ and FNR antibodies, and the asymmetric distribution develops as a consequence of oocyte maturation and is clearly observed by metaphase I. In contrast to the membrane localization of these integrin subunits, α₂, α₅, and VNR are predominantly localized in the cytoplasm of both oocytes and eggs. In the oocyte, each of these integrin subunits is uniformly distributed throughout the cytoplasm. Oocyte maturation is associated with a redistribution of α₅ and VNR, leading to an asymmetric cytoplasmic distribution with an increased localization towards the spindle. α(v), which is localized in the plasma membrane/cortex of both oocytes and eggs, does not show such a change during oocyte maturation. Results of these experiments are discussed in the context of a role for integrins in mediating sperm plasma membrane-egg plasma membrane interactions leading to egg activation.