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Fig. 1 Comparison between fish and tetrapods microsomal triglyceride transfer protein (MTP) large subunit predicted amino acid sequences. Amino acid sequences are numbered from the initiator methionine for human (Hum), pig (Pig), mouse (Mou), chicken (Chi), Fugu (Fug), and zebrafish (Zeb) sequences. Xenopus (Xen) and trout (Tro) sequences are numbered according to the amino acid sequence deduced from the partial cDNA sequences currently available. Residues identical or considered conserved with human sequence are colored in cyan and yellow, respectively. The allowed conservative substitutions were defined as follows: A, G; S, T; E, D; R, K, H; Q, N; V, I, L, M; Y, F; W; P; C. A: Zebrafish MTP large subunit predicted amino acid sequence aligned with the corresponding human sequence. The predicted signal peptide sequences are underlined. Human MTP-predicted structure based on the atomic coordinates of lamprey lipovitellin (Mann et al.,[1999]; Read et al.,[2000]) is shown with arrows indicating β-strands, cylinders depicting α-helices, plain lines representing loops, and dashed lines the unassigned coordinates. Human MTP large subunit is structurally divided into four domains: the N-sheet (β1-β13) consists of amino acids 22 to 303, the α-helical domain (α1-α17) is distributed from amino acids 304 to 598, the C-sheet (C-β1-C-β6) corresponds to the fragment between amino acids 603 and 712, and the A-sheet (αA-A-β4) includes amino acids 725 to 829. Downward and upward black arrowheads indicate intron positions in human and zebrafish MTP large subunit gene, respectively. The number below the zebrafish sequence and attached to the upward black arrow is indicative of the intron number. Gaps inserted to optimize alignments are indicated by dashes. B: Comparison of the C-terminal part of vertebrate MTP large subunit protein sequences encompassing helices αA and αB of the A-sheet domain of human MTP large subunit. Single dots in the trout sequence indicate an unknown sequence. Numbers in parentheses in the alignments indicate the length of the omitted sequence. Arrows below the sequences indicate two amino acid positions critical for the lipid transfer activity of human MTP large subunit (Read et al.,2000]). The first position corresponding to leucine-734 is conserved in the other vertebrate sequences, whereas the second position corresponding to valine-782 is not.