Cloning and Characterization of the First Cnidarian ADP-Ribosylation Factor, and Its Involvement in the Aiptasia-Symbiodinum Endosymbiosis

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Abstract
Marine cnidarian-microalgal endosymbiosis is a form of intracellular association that contributes greatly to the high primary productivity of reefs; however, little is known about its molecular mechanisms. Since the ADP-ribosylation factor (ARF) family proteins are key regulators of host intracellular vesicle transport systems, which are critical to many endosymbiotic interactions, we set out to clone and characterize ARF proteins in the symbiotic sea anemone Aiptasia pulchella. Experiments indicated that at least 3 ARF protein classes (class I, class II and class III) were present and expressed as a single messenger RNA species in Aiptasia, with highest mRNA expression levels for apARF1, medium for apARF5, and lowest for apARF6. Quantitative analysis revealed a great reduction at both the RNA and the protein levels in apARF1, but not apARF5 and apARF6, in the symbiotic animals. The apARF1 protein was highly homologous in sequence to other known ARF1 proteins and displayed a Golgi-like localization pattern. Overall, our study identified apARF1 as a potential negative regulator of Aiptaisia-microalgal endosymbiosis.