BAK1

BAK1 Full Name

BCL2-antagonist/killer 1

BAK1 Introduction

BAK1 (BCL-2 antagonist/killer 1) is a pro-apoptotic protein belonging to the BCL-2 family, which plays a central role in the regulation of programmed cell death (apoptosis). The gene encoding BAK1 is located on human chromosome 6p21.31 and produces a protein of approximately 211 amino acids with a molecular weight of about 23-25 kDa. BAK1 is widely expressed in various human tissues, including the heart, brain, liver, kidney, and immune cells. Unlike some BCL-2 family members that are primarily cytoplasmic, BAK1 is localized to the outer mitochondrial membrane under normal conditions, where it remains inactive through interactions with anti-apoptotic proteins such as BCL-2, BCL-XL, and MCL-1. Upon receiving apoptotic signals, BAK1 undergoes conformational activation, oligomerizes, and forms pores in the mitochondrial membrane, leading to the release of cytochrome c and other apoptogenic factors that ultimately trigger cell death. BAK1 is essential for the intrinsic (mitochondrial) pathway of apoptosis.

Figure 1. Strcuture of BAK1.

Structural Domains and Activation Mechanism
BAK1 shares the characteristic structural architecture of the BCL-2 family, containing four BCL-2 homology (BH) domains designated BH1, BH2, BH3, and BH4, along with a C-terminal transmembrane domain that anchors the protein to the outer mitochondrial membrane. The BH3 domain, in particular, is critical for both activation and inhibition of BAK1. Under steady-state conditions, BAK1 adopts a folded, inactive conformation where the hydrophobic BH3 domain is buried within a groove formed by the BH1, BH2, and BH3 domains. Anti-apoptotic BCL-2 family members (such as BCL-2, BCL-XL, and MCL-1) bind to BAK1 and maintain it in this inactive state. Upon exposure to cellular stress signals (such as DNA damage, growth factor withdrawal, or endoplasmic reticulum stress), BH3-only activator proteins (such as BID, BIM, and PUMA) bind to BAK1, inducing a conformational change that exposes the BH3 domain and triggers BAK1 activation. Activated BAK1 undergoes homooligomerization to form a pore-like structure in the mitochondrial outer membrane.

Role in the Intrinsic Apoptotic Pathway
BAK1, together with its close homolog BAX, is considered an essential executioner of the intrinsic apoptotic pathway. Upon activation, BAK1 and BAX insert into the outer mitochondrial membrane and oligomerize to form macropores (sometimes referred to as mitochondrial apoptosis-induced channels, MACs). These pores allow the release of multiple apoptogenic factors from the mitochondrial intermembrane space into the cytosol. The most critical of these factors is cytochrome c, which binds to apoptotic protease-activating factor 1 (APAF1) to form the apoptosome, a large multi-protein complex that activates caspase-9. Caspase-9 then activates downstream effector caspases (caspase-3, -6, and -7), which cleave numerous cellular substrates and orchestrate the stereotypical morphological and biochemical changes of apoptosis. Second mitochondria-derived activator of caspases (SMAC/DIABLO) and Omi/HtrA2 are also released from mitochondria, where they neutralize inhibitor of apoptosis proteins (IAPs) and further promote caspase activation.

Alternate Names for BAK1

BAK1; BCL2-antagonist/killer 1; BAK; CDN1; BCL2L7; BAK-LIKE; bcl-2 homologous antagonist/killer; bcl2-L-7; BCL2-like 7 protein; bcl-2-like protein 7; apoptosis regulator BAK; pro-apoptotic protein BAK;