ApRab3, a biosynthetic Rab protein, accumulates on the maturing phagosomes and symbiosomes in the tropical sea anemone, Aiptasia pulchella

Symbiosome biogenesis and function are central to the endosymbiotic interaction between symbiotic dinoflagellates and their host cnidarians. To understand these important organelles, we have been conducting studies to identify and characterize symbiosome-associated proteins of the Rab family, key regulatory components of vesicular trafficking and membrane fusion in eukaryotic cells. Our prior studies have implicated three endocytic Rab proteins in the regulation of symbiosome biogenesis. Here, we show that ApRab3 is a new member of the Rab3 subfamily, associating with symbiosomes and accumulating on the maturing phagosomes in the A. pulchella digestive cells. ApRab3 is 78% identical to human Rab3C, and contains all Rab 3-specific signature motifs. EGFP-ApRab3-labeled vesicular structures tended to either align along the cell peripheral, or aggregate at one side of the nucleus. ApRab3 specifically co-distributed with the TGN marker, WGA, but not other organelle-specific markers tested. Immunofluorescence staining with a specific peptide antibody showed similar results. Significantly, an expression of a constitutively active mutant caused the enlargement and random dispersion of EGFP-ApRab3-decorated compartments in PC12 cells. Together, these data suggest that ApRab3 is a new member of the Rab3 subfamily, participating in the biosynthetic trafficking pathway, and symbiosome biogenesis involves an interaction with ApRab3-positive vesicles.