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The Journal of Immunology, 2001, 166: 1710-1715.
Copyright © 2001 by The American Association of Immunologists

Cloning and Characterization of Integrin {alpha} Subunits from the Solitary Ascidian, Halocynthia roretzi1

Seita Miyazawa*, Kaoru Azumi{dagger} and Masaru Nonaka2,*

* Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan; and {dagger} Department of Biochemistry, Graduate School of Pharmaceutical Sciences, Hokkaido University, Hokkaido, Japan

Recent molecular and biochemical analysis has revealed the presence of an opsonic complement system in the solitary ascidian, Halocynthia roretzi, composed of at least C3, two mannan binding protein-associated serine proteases, and factor B. To elucidate further the structure and function of this apparently primitive complement system in the urochordates, we looked for the ascidian complement receptor type 3 (CR3), or type 4 (CR4), which are members of the leukocyte integrin family in mammals. Using degenerate primers, we isolated two integrin {alpha} subunits ({alpha}Hr1 and {alpha}Hr2) from the hemocyte mRNA of H. roretzi, by RT-PCR, and the entire coding sequence of {alpha}Hr1 was determined from cDNA clones. {alpha}Hr1 contains an I domain, the inserted domain characteristic of a subset of mammalian {alpha} subunits, including the leukocyte integrin family. A phylogenetic tree constructed for the {alpha} subunits also supports the ancestral position of {alpha}Hr1 in the monophyletic cluster of I domain-containing {alpha} integrins. The {alpha}Hr1 gene shows hemocyte-specific expression on Northern blot analysis. Western blot analysis and immunocytochemical staining of the hemocytes of H. roretzi using anti-{alpha}Hr1 Ab showed that {alpha}Hr1 subunits exist on the surface of a subpopulation of phagocytic hemocytes. Furthermore, anti-{alpha}Hr1 Ab inhibited C3-dependent phagocytosis, but not basic phagocytosis, of yeast cells by ascidian hemocytes. These observations strongly suggest that {alpha}Hr1 constitutes an integrin molecule on the hemocytes of H. roretzi that functions as an ancestral form of CR3 and CR4 and mediates phagocytosis in the primitive complement system of the ascidian.




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