ApRab11, a cnidarian homologue of the recycling regulatory protein Rab11, is involved in the establishment and maintenance of the Aiptasia–Symbiodinium endosymbiosis

Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis

“…The Rab GTPases are important regulators of vesicular trafficking including in professional phagocytes such as macrophages (Schwartz et al, 2007). Recent studies on the differential localization of three Rab homologs in Aiptasia pulchella symbiont-containing-host cells in healthy hosts compared with stressed hosts suggest that stress results in changes in lysosomal maturation and targeting of symbiosomes for fusion with lysosomes presumably resulting in symbiont digestion (Chen et al, 2005). In addition, there is early evidence in Aiptasia pallida that autophagy, a cellular pathway that controls organelles, vacuoles and tissue homeostasis, could be playing a role in bleaching (Dunn et al, 2007).…”

Symbiophagy as a cellular mechanism for coral bleaching

“…However, during a stress event that results in the degradation of the zooxanthellae, ApRab11 and especially ApRab7 were associated with endocytic phagosomes containing dead or dysfunctional zooxanthellae. 13,14 These findings provide key steps in developing an autophagic mechanism for cnidarian bleaching as both Rab11 (a small GTPase) and Rab7 are recognized markers of autophagic activity across taxa. Xenophagy, the digestion of potential intracellular pathogens, utilizes much of the same autophagic machinery, including the Rab proteins, for the digestion of these pathogens.…”

“…13,14 They reported that the Aiptasia Rab11 is excluded from the vacuole body containing zooxanthellae under normal conditions. However, during a stress event that results in the degradation of the zooxanthellae, ApRab11 and especially ApRab7 were associated with endocytic phagosomes containing dead or dysfunctional zooxanthellae.…”

Zooxanthellae: The Yellow Symbionts Inside Animals

“…This spatial arrangement may support the diffusion and transport of CO 2 , bicarbonate ions, and nutrients from the environment to the algae (Muscatine et al 1998). It could be that the symbiosome membranes also protect the symbionts from being digested (Chen et al 2005). However, it should be noted that the symbiosome does not block the effects of the host-release factor (HRF), which stimulates photosynthate release, and the photosynthesis inhibiting factor (PIF) (Grant et al 2003, see below).…”