A Porcine Homolog of Human Integrin α6 is a Differentiation Antigen of Granulosa Cells1

To search for new molecules relating to the mechanism(s) of follicular differentiation, we raised a new monoclonal antibody (mAb), POG-2, which reacted with the surface of porcine ovarian granulosa cells. An immunohistological study revealed that the antigen appeared initially on the primary follicles. The antigen expression increased during follicular development, reached the highest level in small follicles 1-2 mm in diameter, and then decreased in the preovulatory follicles. The antigen was not detected on luteal cells in the CL. SDS-PAGE profiles of POG-2 antigen purified from isolated porcine granulosa cells by immunoaffinity chromatography indicated that it consisted of two proteins (120 and 130 kDa) and that the antigenic epitope of POG-2 was on the 120-kDa protein. We next purified a large amount of POG-2 antigen from the homogenate of porcine ovaries by immunoaffinity chromatography. Analysis of the partial amino acid sequence of the 120-kDa protein showed that 16 amino acids from the amino terminus were very similar to those of both human and murine mature integrin alpha 6, with 88% homology. SDS-PAGE profiles revealed that the 120-kDa protein was bound to a 28-30-kDa protein by a disulfide bond; this was compatible with the reported characteristics of human integrin alpha 6. We then examined the immunoreactive distribution of integrin alpha 6 using anti-integrin alpha 6 mAb GoH3. The staining profiles obtained with GoH3 were very similar to those obtained with POG-2. These findings showed that the molecule detected by POG-2 was a porcine homolog of human integrin alpha 6 and that integrin alpha 6 was a differentiation antigen of porcine granulosa cells, suggesting an involvement of integrins in follicular differentiation.