Abstract
The cellular and molecular-scale processes underlying the stability of coral-Symbiodinium endosymbioses remain unclear despite decades of investigation. As the coral gastroderm is the only tissue layer characterized by this unique symbiotic association, the membranes of these symbiotic gastrodermal cells (SGCs) may play important roles in the initiation and maintenance of the endosymbiosis. In order to elucidate the interactions between the endosymbiotic dinoflagellates and their coral hosts, a thorough characterization of SGC membranes is therefore required. Cell surface proteins of isolated SGCs were biotinylated herein by a cell impermeant agent, biotin-XX sulfosuccinimidyl ester. The in situ distribution of these biotinylated proteins was uncovered by both fluorescence and transmission electron microscopic imaging of proteins bound to Alexa Fluor® 488-conjugated streptavidin. The identity of these proteins was then determined by two-dimensional gel electrophoresis followed by liquid chromatography-tandem mass spectrometry. Nineteen (19) proteins were identified, and they are known to be involved in the molecular chaperone/stress response, cytoskeletal remodeling, and energy metabolism. These results not only reveal the molecular characters of the host SGC membrane, but also provide critical insight into understanding the possible role of host membranes in this ecologically important endosymbiotic association.
Highlights
The coral-Symbiodinium endosymbiosis is a unique phenomenon in which a phototrophic dinoflagellate lives within the gastrodermal cell of the coral host [1,2]
This endosymbiosis is responsible for the construction of coral reefs across Earth’s tropical seas [1], though the processes involved in its regulation are poorly understood
Cell biology approaches have attempted to elucidate four processes that are integral to the biology of these associations: (i) recognition [2,3] and phagocytosis [4,5] of Symbiodinium into host symbiotic gastrodermal cells (SGCs); (ii) regulation of host cell growth and proliferation of the endosymbionts; (iii) metabolic exchanges and the nutrient dialogue between Symbiodinium and their host cells; and (iv) host coral calcification [6,7]
Summary
The coral-Symbiodinium endosymbiosis is a unique phenomenon in which a phototrophic dinoflagellate (i.e., the endosymbiont) lives within the gastrodermal cell of the coral host [1,2]. This endosymbiosis is responsible for the construction of coral reefs across Earth’s tropical seas [1], though the processes involved in its regulation are poorly understood. After the phagocytosis of the Symbiodinium into the host gastrodermal cells, a symbiosome membrane is enveloped around the endosymbionts [8,9,10]. 17 symbiosome membrane-associated proteins have been identified, and they include membrane receptors involved in cell recognition, as well as proteins involved in cytoskeletal remodeling, ATP synthesis/proton homeostasis, transport, the stress response, and prevention of apoptosis [9]
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