Mass spectrometry of lens fiber membrane proteins
Shearer, David B.
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Gap junctions are communicating junctions between cells that allow small molecules to pass from the cytoplasm of one cell to the cytoplasm of an adjacent cell. The pores of gap junctions are comprised of two adjacent connexons on neighboring cells, and each connexon is comprised of six connexin proteins. The eye lens of vertebrates is an avascular tissue that is dependent on gap junctions for the distribution of nutrients as well as the removal of waste products. In addition, as the lens cells develop into fibers, they lose their intracellular organelles including the membrane-bound organelles, and are highly dependent on connexons for movement of metabolites and waste materials. Only two connexins, in Bos Taurus Cx44 and Cx49, are highly expressed in lens fiber cells. Thus, the lens offers an excellent system for studying gap junctions. In this study, high-pressure liquid chromatography (HPLC) and mass spectrometry (MS) techniques were used to isolate and characterize connexin proteins from the eye lens of the cow and mouse. Despite over 300 proteins being identified from bovine lens using MS techniques, it was still possible to identify the two connexin proteins following proteolytic digests and MS analysis of the resultant peptides. Several post- translational modifications (PTMs) were identified and characterized in lens fiber connexins, including phosphorylations, acetylations and deamidations and proteolytic cleavages. Changes in phosphorylation of several other lens proteins upon the activation of protein kinase C were also identified. Detection of the orthologous proteins in mouse lens proved more challenging, but peptides derived from both connexin proteins were also detected from this tissue and PTMs of mouse connexins were also observed. Glutathione-S-transferase fusions to mouse Cx44 and Cx50 were used to identify a number of proteins that may interact with the mouse connexins, and the relevance of those interactions was considered. The utility of mass spectrometry to the identification of specific proteins from complex mixtures was clearly demonstrated, and its application to understanding the functional relevance of PTMs was discussed.