Immunocytochemical Localization of Rab5 in Perialgal Vacuoles of a Freshwater Sponge

Abstract

Chlorella sp. is a green alga that exists in endosymbiotic relationships with a number of freshwater invertebrates including the sponge, Ephydatia muelleri. Algae enter sponge cells through phagocytosis, but unlike most cells taken up by this process, the algal cells are not degraded. The mechanism by which Chlorella escapes degradation is currently unknown. One hypothesis is that the vacuoles containing algal cells do not progress along the degradative pathway but instead are maintained in an immature, pre-lysosomal state, displaying the early endosomal marker Rab5. In order to test this hypothesis, I localized Rab5 in sponge cells using immunocytochemistry. Two antibodies against human Rab5 were first shown by Western blotting to bind to a sponge protein having a molecular weight similar to that of mammalian Rab5. Both antibodies also recognized some higher molecular weight proteins. From these results, I concluded that there was sufficient cross reactivity to use the antibodies for immunocytochemistry. I was unable to localize Rab5 in cells using one of the antibodies; however, with the other, I detected antibody bound to algae-containing vacuoles. This latter result suggests that vacuoles containing symbiotic algae may express Rab5 and that maturation of the vacuoles may be hindered.