Immunogold localization of photosystem I and photosystem II light-harvesting complexes in cryptomonad thylakoids

Summary— The molecular organization of the thylakoids of Cryptomonas rufescens was studied by immunoelectron microscopy employing antibodies against photosystem (PS)-I and two PS-II antenna proteins. The PS-I complex and the 19-kDa chlorophyll a/c light-harvesting (LH) protein are both localized along the length of the thylakoid membranes. The external membranes of the paired thylakoids are enriched in PS-I whereas the chlorophyll a/c LH protein is more concentrated in the internal or appressed membranes. However, unlike the situation in higher plants and Chlamydomonas, there is not a marked asymmetry in the concentration of PS-I and chorophyll a/c LH protein in the two types of membranes. Double labelling studies of sections and isolated PE-PS-II particles with anti-phycoerythrin and anti-LH confirmed that phycoerythrin is localized in the thylakoid lumen and that this pigment exists in two forms, a fraction closely associated with the thylakoid membranes and another fraction free in the lumen. These results confirm the uniqueness of cryptomonad thylakoids.