Localization of the Late Endosome Marker Rab7 in the Freshwater Sponge Ephydatia muelleri

Abstract

The green alga, Chlorella, exists in a symbiotic relationship with the freshwater sponge, Ephydatia muelleri. Chlorella is taken up by phagocytosis and incorporated into phagosomes. However, the phagosomes containing the algae fail to fuse with lysosomes, resulting in a sustained symbiotic relationship. One possible explanation for the retention of algae in non-lysosomal vacuoles is that the phagosomes do not acquire the surface proteins needed for fusion with lysosomes. Among these proteins is Rab7, which has been suggested to regulate the transition from phagosome to lysosome. I hypothesized that the vacuole containing the symbiotic algae is early endosome-like and thus Rab7 is not present on the algal-containing vacuole membrane. Immunocytochemistry with antibody directed against human Rab7 was used in an attempt to localize Rab7 in the sponge cells. Western blot analysis was used to establish the specificity of the antibody. The results of both the Western blots and immunocytochemistry were inconsistent. Banding in the Western blots was faint, and the pattern of staining varied with the concentration of the primary antibody. Similarly, localization of Rab7 by immunocytochemistry was inconclusive. The inconsistency in both the Western blots and immunocytochemistry indicates that more research is needed to clarify the role Rab7 plays in establishing the symbiotic relationship between algae and sponge.