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The EGF-TM7 family (CD97 and EMR1) is a group of class II seven-span transmembrane receptors predominantly expressed by cells of the immune system. Recently, we have identified CD55, a regulatory molecule of the complement cascade, as a cellular ligand of human CD97 (hCD97). In this study, the molecular properties of mouse CD97 (mCD97) are described. Like hCD97, mCD97 has an extended extracellular region with several epidermal growth factor-like (EGF) domains. Due to alternative RNA splicing, isoforms with three and four EGF domains exist, designated mCD97(EGF1,2,4) and mCD97(EGF1,2,3,4) respectively. All EGF domains, except for the N-terminal one, possess a calcium-binding site. In a third isoform mCD97(EGF1,2,X,3,4), a sequence of 45 amino acids was found between the second and third EGF domain that does not correspond to any known protein module. Using newly generated mCD97 mAb, we show that analogous to the blood expression pattern of hCD97, mCD97 can be found on lymphoid and myeloid cells. Adhesion of mouse erythrocytes and splenocytes to COS cells expressing mCD97(EGF1,2,4) or mCD97(EGF1,2,3,4) could be blocked by mouse CD55 (mCD55) antibody, identifying mCD55 as a cellular ligand for mCD97. Consistent with the necessity of directly linked EGF domains for the integrity of the CD55-binding site on hCD97, no adhesion was detected to the largest mouse isoform mCD97(EGF1,2,X,3,4). Remarkably, we found that the interaction between CD97 and CD55 is phylogenetically restricted, as indicated by the selective adhesion of primate erythrocytes to hCD97 transfectants, and of mouse and rat erythrocytes to mCD97 transfectants respectively.

The EGF-TM7 family (CD97 and EMR1) is a group of class II seven-span transmembrane receptors predominantly expressed by cells of the immune system. Recently, we have identified CD55, a regulatory molecule of the complement cascade, as a cellular ligand of human CD97 (hCD97). In this study, the molecular properties of mouse CD97 (mCD97) are described. Like hCD97, mCD97 has an extended extracellular region with several epidermal growth factor-like (EGF) domains. Due to alternative RNA splicing, isoforms with three and four EGF domains exist, designated mCD97(EGF1,2,4) and mCD97(EGF1,2, 3,4) respectively. All EGF domains, except for the N-terminal one, possess a calcium-binding site. In a third isoform mCD97(EGF1,2,X,3, 4), a sequence of 45 amino acids was found between the second and third EGF domain that does not correspond to any known protein module. Using newly generated mCD97 mAb, we show that analogous to the blood expression pattern of hCD97, mCD97 can be found on lymphoid and myeloid cells. Adhesion of mouse erythrocytes and splenocytes to COS cells expressing mCD97(EGF1,2,4) or mCD97(EGF1,2, 3,4) could be blocked by mouse CD55 (mCD55) antibody, identifying mCD55 as a cellular ligand for mCD97. Consistent with the necessity of directly linked EGF domains for the integrity of the CD55-binding site on hCD97, no adhesion was detected to the largest mouse isoform mCD97(EGF1,2,X,3,4). Remarkably, we found that the interaction between CD97 and CD55 is phylogenetically restricted, as indicated by the selective adhesion of primate erythrocytes to hCD97 transfectants, and of mouse and rat erythrocytes to mCD97 transfectants respectively.