MADD is highly homologous to a Rab3 guanine-nucleotide exchange protein (Rab3-GEP)

The MADD protein (death domain MAP kinase activator) has recently been identified as a death-domain-containing protein involved in the activation of mitogen-activated protein (MAP) kinase and arachadonic acid release mediated by the tumor necrosis factor receptor type 1 (TNFR1) [1xMADD, a novel death domain protein that interacts with the type I tumor necrosis factor receptor and activates mitogen-activated protein kinase. Schievella, AR, Chen, JH, Graham, JR, and Lin, L. J Biol Chem. 1997; 272: 12069–12075Crossref | PubMed | Scopus (119)See all References[1]. The death domain of MADD (amino acids 1281–1356) can bind to the death domain of TNFR1 and can activate MAP kinases. Overexpression of MADD has no effect on the induction of apoptosis, however. Thus, although the presence of a death domain is suggestive of a potential protein–protein interaction, it does not always implicate a protein in the apoptotic process. Sequence analysis of the death domain in the MADD protein was performed using the BLAST local alignment program [2xBasic local alignment search tool. Altschul, SF, Gish, W, Miller, W, Myers, EW, and Lipman, DJ. J Mol Biol. 1990; 215: 403–410PubMed | Scopus (0)See all References[2] to search the non-redundant sequence GenBank database at NCBI. Two proteins were identified that had almost identical amino-acid sequences to that of the death domain of MADD (Figure 1Figure 1). Analysis of the full-length protein sequences of these two proteins using MacVector (Eastman Kodak) and Geneworks (Intelligenetics) protein alignment programs revealed that one of them was the GDP/GTP exchange protein Rab3-GEP [3xIsolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily of small G proteins. Wada, M, Nakanishi, H, Satoh, A, Hirano, H, Obaishi, H, Matsuura, Y, and Takai, Y. J Biol Chem. 1997; 272: 3875–3878Crossref | PubMed | Scopus (125)See all References[3]. Rab3-GEP has greater than 94% overall identity to MADD. The only significant difference between MADD and Rab3-GEP is in a region amino terminal to their death domains, where 21 amino acids have been deleted in MADD. The other protein with an almost identical death domain to that of MADD was a brain protein of unknown function, KIAA0358. Furthermore, full-length MADD and Rab3-GEP are 87% identical to KIAA0358, although KIAA0358 has a carboxy-terminal 61 amino-acid deletion.Figure 1MADD-related isoforms. Blue boxes identify the position of the death domain present in Rab3-GEP (GenBank Accession number U72995), KIAA0358 (AB002356), MADD (U77352) and DENN (U44953).View Large Image | View Hi-Res Image | Download PowerPoint SlideSequence analysis of the amino-terminal region of the MADD protein identified a fourth protein, DENN, which is highly homologous to MADD (98% identity), Rab3-GEP and KIAA0358, although it lacks the carboxy-terminal 300 amino acids of MADD, including the death domain. DENN was previously identified as a differentially expressed protein in neoplastic tissues [4xDENN, a novel human gene differentially expressed in normal and neoplastic cells. Chow, VT and Lee, SS. DNA Seq. 1996; 6: 263–273PubMedSee all References[4] and shown to have homology to Rab3-GEP [3xIsolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily of small G proteins. Wada, M, Nakanishi, H, Satoh, A, Hirano, H, Obaishi, H, Matsuura, Y, and Takai, Y. J Biol Chem. 1997; 272: 3875–3878Crossref | PubMed | Scopus (125)See all References[3] and the product of the Caenorhabditis elegans aex-3 gene [5xAex-3 encodes a novel regulator of presynaptic activity in C. elegans. Iwasaki, K, Staunton, J, Saifee, O, Nonet, M, and Thomas, JH. Neuron. 1997; 18: 613–622Abstract | Full Text | Full Text PDF | PubMedSee all References[5]. The aex-3 gene product has 33% homology to Rab3-GEP, and mutation of aex-3 results in mislocalization of Rab3 [5xAex-3 encodes a novel regulator of presynaptic activity in C. elegans. Iwasaki, K, Staunton, J, Saifee, O, Nonet, M, and Thomas, JH. Neuron. 1997; 18: 613–622Abstract | Full Text | Full Text PDF | PubMedSee all References[5].Rab family members are modified by geranyl–geranyl moieties, cycle between the GDP-bound inactive and GTP-bound active state [3xIsolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily of small G proteins. Wada, M, Nakanishi, H, Satoh, A, Hirano, H, Obaishi, H, Matsuura, Y, and Takai, Y. J Biol Chem. 1997; 272: 3875–3878Crossref | PubMed | Scopus (125)See all References[3], and have been shown to function in vesicular traffic [[3]xIsolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily of small G proteins. Wada, M, Nakanishi, H, Satoh, A, Hirano, H, Obaishi, H, Matsuura, Y, and Takai, Y. J Biol Chem. 1997; 272: 3875–3878Crossref | PubMed | Scopus (125)See all References, [6]xFriends and family: the role of Rab GTPases in vesicular traffic. Novick, P and Brennwald, P. Cell. 1993; 75: 597–601Abstract | Full Text PDF | PubMed | Scopus (272)See all References]. Other small GTPases, such as Ras, Rac and Cdc42 [[7]xSelective activation of the JNK signaling cascade and c-jun transcriptional activity by the small GTPases Rac and CDC42Hs. Minden, A, Lin, A, Claret, F-X, Abo, A, and Karin, M. Cell. 1995; 81: 1147–1157Abstract | Full Text PDF | PubMed | Scopus (1368)See all References, [8]xThe small GTP-binding proteins Rac1 and Cdc42 regulate the activity of the JNK/SAPK signaling pathway. Coso, OA, Chiariello, M, Yu, J-C, Teramoto, H, Crespo, P, Xu, N et al. Cell. 1995; 81: 1137–1146Abstract | Full Text PDF | PubMed | Scopus (1460)See all References, [9]xHow MAP kinases are regulated. Cobb, MH and Goldsmith, EJ. J Biol Chem. 1995; 270: 14843–14846Crossref | PubMed | Scopus (1562)See all References], have been shown to play important roles in positive and negative regulation of cell signaling and growth. The identification of MADD as a regulator of TNFR1 function (through its death domain), and the similarity between MADD and Rab3-GEP provides a connection between TNFR1 activation and downstream kinase activity through a guanine-nucleotide exchange protein. The variations that exist among the four identified proteins may have arisen as a result of alternative splicing, thereby generating different isoforms of death-domain-containing proteins. Future studies may determine whether these MADD/Rab3-GEP isoforms are capable of activating specific MAP kinases via GDP/GTP exchange and of effecting TNF-mediated signaling, arachadonic acid release and/or vesicular traffic.