Homologies between gap junction proteins in lens, heart and liver

The cells in the mammalian lens are electrically and metabolically coupled with each other by a network of gap junctions. These are clusters of transmembrane channels by which the fibre cells situated deeper in the lens communicate through the epithelium with the aqueous humour, the source of nutrients for the lens. Hence gap junctions are important for lens transparency. The gap junction proteins in the mammalian lens have not yet been identified with certainty. A putative fibre gap junction protein of relative molecular mass 26,000 (26K) is not related to those from other tissues, such as the liver 28K junction component. Another lens membrane protein with Mr 70K (MP70) has also been localized in the lens fibre gap junctions. Here we demonstrate by amino-terminal sequence analysis that MP70 and its in vivo-processed form, MP38 (ref. 8), belong to a wider family of gap junction proteins. With this new data on the lens, homologies between gap junction proteins now extend to organs derived from all three embryonal layers, endoderm (liver), mesoderm (heart) and ectoderm (lens).