Orthologues
Discussing overall structure and implication of MSA in orthologues
2BID
Fig. 1. 2BID. Hydrophobic regions (helices 6 and 7) coloured orange. Helices 1-5 and 8 coloured blue. Loop regions coloured magenta.
Fig. 2. 2BID. Conserved regions between species and isoforms coloured. Non-conserved regions grey.
Fig. 3. tBID. The 2BID protein is labeled as in Fig. 1, but with the N-terminal section of the cleaved protein coloured black. The C-terminal structures coloured as in Fig. 1 are those that comprise tBID.
2BID is the human pro-apoptotic protein. Apart from two extra amino acid residues, glycine and serine, that are added N-terminal to its starting methionine as a result of thrombin cleavage, 2BID is identical to isoform 2 of BID [1]. 2BID has nine loop regions and eight corresponding helices. Helices α6 and α7 display hydrophobic activity and are consequently found in the centre of the protein, surrounded by the other helices (Fig. 1).
When compared to the other isoforms of BID, the areas of highest conservation are in loop regions six, seven, and eight, and helices α6 and α7 (Fig. 2). This region has perfect conservation from residue 165-184 (Table 1). The hydrophobic region is necessary for apoptotic function as it is the region of tBID that embeds in the outer mitochondrial membrane (OMM) [2].
When BID is cleaved by caspase-8 in the apoptotic pathway, the truncated C-terminal fragment, tBID, is able to embed itself in the OMM. tBID is then able to recruit Bax to the OMM as well, where Bax embeds itself, driving the permeabilisation of the membrane [3]. 2BID is identical in structure to tBID from residue 61 onwards, indicating residue 60 as the cleavage site (Fig. 3).
The BH3 region of BID (amino acid residues 90-98) is also highly conserved, and is found in the region of helix α3 [4]. After cleavage of BID and translocation of tBID into the OMM, helix α3 is very exposed; the BH3 domain in this helix is responsible for interacting with Bax to recruit it to the membrane.
Table 1. Species Comparison. Protein conservation between species. Areas of high conservation are noted with an asterisk.