Immunogold localization of a transglutaminase related to grana developmentin different maize cell types

A comparative study of the subcellular localization of a plant transglutami nase (TGase: EC 2.3.2.13) in various in vivo and in vitro maize cell types was carried out with a polyclonal antibody raised against a 58 kDa TGase pu rified from Helianthus tuberosus leaves. Immunocytochemical staining, follo wed by electron microscopy, showed that this enzyme was markedly present in the grana-appressed thylakoids of mature chloroplasts of the light-exposed cells, Moreover, during embryogenic callus chloroplast differentiation, th e abundance of TGase in the grana-appressed thylakoids depended on the degr ee of grana development and was greater than in mature leaf chloroplasts. I n addition to the 58 kDa form, two other forms of the protein (of 77 and 34 kDa) were obtained by Western blot. The 77 kDa form might correspond to th e inactive form and was immunodetected in dense vesicles observed in dark-g rown embryogenic callus cells. In adult leaves, the enzyme was also markedl y present in the grana-appressed thylakoids of the mesophyll cell chloropla sts, though very scarce and dispersed in the bundle-sheath cell chloroplast s (which do not contain grana). The concordance of these localizations with those described for the light-harvesting antenna proteins of the photosyst em II suggests that it is possible that this TGase has a functional role in photosynthesis, perhaps modulating the photosynthetic efficiency and the a bsorption of excess light by means of polyamine conjugation to the antenna proteins.