This repeat of unknown function was first identified in Diptera proteins; later on it was found in a wide range of invertebrates and vertebrates [1], such as natterin [2,3]. It is often present in proteins comprising only two or four DM9 repeats and occasionally is linked to other domains at its N- or C-terminal. In the mannose-specific lectin CGL1 from Crassostrea gigas, DM9 exhibits high binding specificity and avidity toward D-mannose residue and served as a pattern recognition receptor (PRR) with a broad range of recognition spectrum to various pathogen-associated molecular patterns [4]. It also mediates immune recognition and cellular encapsulation [5].

1: DM9 Domain Containing Protein Functions As a Pattern Recognition Receptor with Broad Microbial Recognition Spectrum. Jiang S, Wang L, Huang M, Jia Z, Weinert T, Warkentin E, Liu C, Song X, Zhang H, Witt J, Qiu L, Peng G, Song L; Front Immunol. 2017;8:1607. PMID:29238341

2: Natterins, a new class of proteins with kininogenase activity characterized from Thalassophryne nattereri fish venom. Magalhaes GS, Lopes-Ferreira M, Junqueira-de-Azevedo IL, Spencer PJ, Araujo MS, Portaro FC, Ma L, Valente RH, Juliano L, Fox JW, Ho PL, Moura-da-Silva AM; Biochimie. 2005;87:687-699. PMID:16054523

3: The Natterin Proteins Diversity: A Review on Phylogeny, Structure, and Immune Function. Lima C, Disner GR, Falcao MAP, Seni-Silva AC, Maleski ALA, Souza MM, Reis Tonello MC, Lopes-Ferreira M; Toxins (Basel). 2021; [Epub ahead of print] PMID:34437409

4: Identification, Characterization, and X-ray Crystallographic Analysis of a Novel Type of Mannose-Specific Lectin CGL1 from the Pacific Oyster Crassostrea gigas. Unno H, Matsuyama K, Tsuji Y, Goda S, Hiemori K, Tateno H, Hirabayashi J, Hatakeyama T; Sci Rep. 2016;6:29135. PMID:27377186

5: A DM9-containing protein from oyster Crassostrea gigas (CgDM9CP-3) mediating immune recognition and encapsulation. Liu Y, Wang W, Zhao Q, Yuan P, Li J, Song X, Liu Z, Ding D, Wang L, Song L; Dev Comp Immunol. 2021;116:103937. PMID:33242570