Immunocytochemical Localization of Rab 5 In Freshwater Sponge Cells Containing Algal Endosymbionts

Abstract

The freshwater sponge, Ephydcitia muelleri, harbors symbiotic algae within its cells. These algae are taken up by phagoctyosis, but they fail to be broken down. One explanation for the lack of degradation is that their digestion is prevented at an unknown point along the phagocytic pathway. In an attempt to determine if the vacuoles containing algal endosymbionts had characteristics associated with early endosomes, I used immunocytochemistry to localize Rab 5 in sponge cells. Rab 5 is a protein that binds to membranes and functions in docking and fusion events early in the endocytic and phagocytic pathways. Immunoblots using antibody against human Rab 5 identified three proteins, with molecular weights of 26,000, 31,000, and 46,000, in homogenates of sponge tissue. The first has a molecular weight close to mammalian Rab 5. Immunocytochemical localization revealed deposits of reaction product on the membranes of the perialgal vacuoles and in the cytosol. These results indicate that Rab 5 could be bound to the surface of the vacuoles and that vacuoles housing endosymbiotic algae could be blocked at an early stage in the phagocytic pathway.