The 2M6 antigen is a Müller cell-specific intracellular membrane-associated protein of the sarcolemmal-membrane-associated protein family and is also Top(AP)

PURPOSE: The differentiation marker 2M6 has been used to identify Müller cells within the developing chick retina for several years, although the molecular identity of 2M6 was not known. This study was aimed at determining the identity of the protein antigen recognized by the 2M6 monoclonal antibody. METHODS: Affinity chromatography and subsequent mass spectrometry were used to determine the molecular identity of the 2M6 antigen. Immunohistochemistry of monolayer preparations and paraffin-embedded sections of chick retina were performed to localize expression of the 2M6 antigen within cells of the chick retina. RESULTS: Mass spectrometry analyses revealed that the 2M6 antigen is identical (with 95% probability) to the protein known as Top(AP), which is a member of the sarcolemmal membrane-associated protein family of proteins. The 2M6 polypeptide is expressed by Müller glial cells as well as boundary cells within the chick retina. Expression localizes to intracellular membrane structures within those cells. CONCLUSIONS: Members of the sarcolemmal membrane-associated protein family of proteins have been implicated in structural and functional roles related to the cytoskeleton and Ca(+2) release from internal stores. It is thought that 2M6 plays a similar role in Müller cells of the vertebrate retina.