Overview
There are four isoforms:
Isoform 1 (BID EL - extra-long, 241aa)
Isoform 2 (BID L - long, 195aa)
Isoform 3 (BID S - short, 131aa)
Isoform 4 (BID ES - extra short, 99aa)
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Isoforms 1 and 2 have identical C-terminuses and contain the BH3 domain; therefore, they are likely to induce apoptosis in a similar fashion though their differing N-terminal sequences relate to differing subcellular localisations. When isoform 1 is cleaved, the N-terminal fragment localises to the Golgi apparatus and the C-terminal fragment localises to mitochondria [1].
Isoforms 1, 2, and 3 have an inhibitory BH3-B domain towards the N-terminus, which has a similar sequence to the BH3 domain as shown in the MSA analysis. As isoforms 3 and 4 do not have the BH3 domain, they do not cause dimerisation with other proteins necessary for the pro-apoptotic pathway. The BH3-B domain in isoform 3 can inhibit apoptosis by inhibiting tBIDL (truncated isoform 2) [1]. Tan et al. [2] demonstrated with a pull-down assay, using bacterial GST (glutathione S-transferase) as a fusion protein to be expressed with the BH3-B domain, that GST-BH3-B selectively associated with in vitro translated tBID. It did not associate with Bcl-X or Bax nor full-length BID. Tan et al. postulates that a region in full-length BID is masked from interaction with BH3-B. In vivo tests showed that BH3-B does indeed interact with tBID in mammalian and yeast cells. As such, interactions between peptides containing BH3-B and peptides containing BH3 domains are likely to regulate the function of tBIDL in apoptosis. Isoforms 1 and 2 do not act in such way due to the presence of both domains.
Although the BH3 and BH3-B domains are absent in isoform 4, it may induce apoptosis by targeting the mitochondria in a BH3-independent manner, similar to the mechanism shown in isoform 2 which requires cardiolipin [3]. In the absence of cardiolipin, isoform 4 may inhibit tBID from binding the outer mitochondrial matrix (OMM) and hence it functions in response to its cellular environment.
Below are hyperlinked flat files and sequence of amino acids for the respective isoforms in FASTA format.
FASTA
>NP_932070.1 BH3-interacting domain death agonist isoform 1 [Homo sapiens]
MCSGAGVMMARWAARGRAGWRSTVRILSPLGHCEPGVSRSCRAAQAMDCEVNNGSSLRDECITNLLVFGFLQSCSDNSFRRELDALGHELPVLAPQWEGYDELQTDGNRSSHSRLGRIEADSESQEDIIRNIARHLAQVGDSMDRSIPPGLVNGLALQLRNTSRSEEDRNRDLATALEQLLQAYPRDMEKEKTMLVLALLLAKKVASHTPSLLRDVFHTTVNFINQNLRTYVRSLARNGMD
>NP_001231496.1 BH3-interacting domain death agonist isoform 2 [Homo sapiens]
MDCEVNNGSSLRDECITNLLVFGFLQSCSDNSFRRELDALGHELPVLAPQWEGYDELQTDGNRSSHSRLGRIEADSESQEDIIRNIARHLAQVGDSMDRSIPPGLVNGLALQLRNTSRSEEDRNRDLATALEQLLQAYPRDMEKEKTMLVLALLLAKKVASHTPSLLRDVFHTTVNFINQNLRTYVRSLARNGMD
>sp|P55957-3|BID_HUMAN Isoform 3 of BH3-interacting domain death agonist OS=Homo sapiens GN=BIDMDCEVNNGSSLRDECITNLLVFGFLQSCSDNSFRRELDALGHELPVLAPQWEGYDELQTDGNRSSHSRLGRIEAGASDNNTASAEEETEAAGSVAVERGLHGAATVILKVKKTSSGILPGTSPRSGTAWTVASLRAW
>sp|P55957-4|BID_HUMAN Isoform 4 of BH3-interacting domain death agonist OS=Homo sapiens GN=BIDMDRSIPPGLVNGLALQLRNTSRSEEDRNRDLATALEQLLQAYPRDMEKEKTMLVLALLLAKKVASHTPSLLRDVFHTTVNFINQNLRTYVRSLARNGMD
MSA results of isoforms
The MSA of the four human isoforms is shown (Fig. 1) with the key components indicated.
As described above, isoform 1’s additional sequence of 46 amino acids at the N-terminus can be seen in the MSA. This is then followed by an invariant sequence across isoforms 1, 2, and 3 until position 75 for isoform 3. Within the said invariant sequence, there is the inhibitory BH3-B domain consisting of nine amino acids which can interact with the BH3 domain in truncated forms of the protein. Tan et al. [1] found that the core regions of the BH3 domain, L, G and E (not D), are essential components to the interaction between BH3-B and BH3 domains.
Caspase-8 cleaves at the position indicated by the start of the purple box. As noted, following position 75 for isoform 3, its subsequent sequence largely lacks consensus with isoforms 1 and 2. This is consistent with our understanding that isoform 3 is non-functional in the pro-apoptotic pathway.
Isoform 4 has an extra short sequence of 99 amino acids; it can be seen that its sequence begins towards the C-terminus of the BH3 domains of isoforms 1 and 2. Therefore, isoform 4 lacks the BH3 and BH3-B domains. Nevertheless, its sequence is identical to the remaining parts of isoforms 1 and 2. This also confirms the similarity between isoforms 1 and 2 as noted on this page where the only difference is the extended region in isoform 1.
Fig. 1. MSA of the four human isoforms of BID (IS1-4). The extended N-terminus of IS1 has been highlighted in turquoise. The differently coloured boxes indicate the largely conserved domains: Green marks the BH3-B domain, purple the tBID domain, and red the BH3 domain. IS3 largely lacks consensus from position 75 onwards, while IS4 has an extra short sequence that begins towards the end of the conserved BH3 domain.