Protective antigen constitutes one of the three proteins composing the anthrax toxin; it mediates attachment to host cells and translocation of edema factor (EF) and lethal factor (LF) into the host cytoplasm (PubMed:11700562, PubMed:14507921, PubMed:15243628, PubMed:15326297). PA associated with LF forms the lethal toxin (LeTx) and causes death when injected; PA associated with EF forms the edema toxin (EdTx) and produces edema (PubMed:1651334). PA induces immunity to infection with anthrax (PubMed:11544370). Mediates the attachment to host cells by binding host cell receptors ANTXR1 and ANTXR2 (PubMed:11700562, PubMed:14507921, PubMed:15243628, PubMed:15326297). Following host cell surface attachment, PA is cleaved by FURIN to generate the PA-63 (Protective antigen PA-63) form, which constitutes the mature form of the protein that oligomerizes and forms a pore to translocate the enzymatic toxin components edema factor (EF) and lethal factor (LF) into the host cytosol (PubMed:11700562, PubMed:15243628, PubMed:15326297). Mature form that oligomerizes and forms a pore to translocate the enzymatic toxin components edema factor (EF) and lethal factor (LF) into the host cytosol (PubMed:15243628, PubMed:15326297). Following attachment to host cell receptors and cleavage by FURIN, homooligomerizes to form ring-shaped oligomers that are in a pre-pore conformation, and associates with EF and LF (PubMed:10085027, PubMed:12117959, PubMed:15313199). Toxin-leaded complexes are then endocytosed in a clathrin-dependent process, followed by a conformational change of oligomerized PA-63 from the pre-pore to pore state, which is triggered by the low pH in the endosome (PubMed:10085027, PubMed:12551953, PubMed:20221438, PubMed:15326297). Once active, the pore mediates unfolding of EF and LF, which pass through the pore and translocate into the host cytosol (PubMed:16051798, PubMed:21037566, PubMed:32047164, PubMed:32810181, PubMed:32521227). Interacts with host ANTXR1 and ANTXR2. Homooligomer; homooligomerizes to form homoheptamers (PA-63(7)) or homooctamers (PA-63(8)) (PubMed:10085027, PubMed:16051798, PubMed:19627991, PubMed:20433851, PubMed:25778700, PubMed:32810181). PA-63(7) or PA-63(8) form ring-shaped oligomers that are in a pre-pore conformation, which do not penetrate the host membrane (PubMed:19627991, PubMed:20433851, PubMed:32810181). PA-63(8) displays an enhanced stability, suggesting that this form circulates in the blood to reach and exert toxicity even in distant tissues (PubMed:20433851). Interacts with lethal factor (LF) and edema factor (EF); can bind LF and EF simultaneously and interaction takes place following homooligomerization on the host cell membrane (PubMed:10085027, PubMed:12117959, PubMed:15313199, PubMed:21037566, PubMed:32047164, PubMed:32810181, PubMed:32521227). PA-63(7) homoheptamer interacts with three molecules of LF to form the PA(7)LF(3) complex, in which the relative position of the N-terminal alpha-helices in the three LFs determines which factor is translocated first (PubMed:32810181). Secreted through the Sec-dependent secretion pathway (PubMed:12606539). Therefore, PA is translocated across the membrane in an unfolded state and then it is folded into its native configuration on the trans side of the membrane, prior to its release to the environment (PubMed:12606539). PA requires the extracellular chaperone PrsA for efficient folding (PubMed:12606539). It circulates in the host blood and binds host cell receptors at the cell surface (PubMed:11700562, PubMed:14507921). Following attachment to host cell receptors at the cell surface and cleavage by FURIN, homooligomerizes to form ring-shaped oligomers that are in a pre-pore conformation, and associates with EF and LF (PubMed:15313199). Loaded complexes are then endocytosed in a clathrin-dependent process, followed by a conformational change of oligomerized PA-63 from the pre-pore to pore state, which is triggered by the low pH in the endosome (PubMed:10085027, PubMed:15326297). The molecule is folded into four functional domains (PubMed:1651334, PubMed:9039918). Each domain is required for a particular step in the toxicity process (PubMed:1651334). Domain 1 contains two calcium ions and the proteolytic activation site (PubMed:1651334). Cleavage of the PA monomer releases the subdomain 1a, which is the N-terminal fragment of 20-kDa (PA-20) (PubMed:8051159, PubMed:11207581, PubMed:9039918). The subdomain 1b is part of the remaining 63-kDa fragment (PA-63) and contains the binding sites for LP and EF (PubMed:8051159, PubMed:11207581, PubMed:9039918). Domain 2 is a beta-barrel core containing a large flexible loop that has been implicated in membrane insertion and pore formation (PubMed:1651334, PubMed:11356563, PubMed:9039918). There is a chymotrypsin cleavage site in this loop that is required for toxicity (PubMed:1512256, PubMed:7961869, PubMed:9039918). Domain 3 has a hydrophobic patch thought to be involved in protein-protein interactions (PubMed:1651334, PubMed:11222612, PubMed:9039918). Domain 4 appears to be a separate domain and shows limited contact with the other three domains: it would swing out of the way during membrane insertion (PubMed:1651334, PubMed:10085028, PubMed:12771151, PubMed:9039918). It is required for binding to the receptor; the small loop is involved in receptor recognition (PubMed:1651334, PubMed:10085028, PubMed:12771151, PubMed:9039918). Phe-456 residue forms the phi-clamp in the pore and catalyzes protein translocation via a charge-state-dependent Brownian ratchet (PubMed:16051798, PubMed:25778700). During conversion of the heptameric pre-pore precursor to the pore, the seven Phe-427 residues converge within the lumen to generate the narrowest point in the channel lumen (6 Angstroms in width) (PubMed:16051798, PubMed:25778700). To pass through this hydrophobic restriction, substrate proteins LF and EF need to be unfolded prior to translocation (PubMed:25778700). The alpha-clamp consists in an amphipathic cleft between two adjacent PA protomers, which assists the unfolding of substrate proteins LF and EF (PubMed:21037566, PubMed:32047164, PubMed:32521227). The alpha-clamp binds non-specifically to alpha-helices of substrate proteins LF and EF (PubMed:21037566, PubMed:32047164, PubMed:32521227). Proteolytic activation by FURIN cleaves the protein in two parts, PA-20 and PA-63; the latter is the mature protein (PubMed:1644824, PubMed:1438214, PubMed:8051159, PubMed:11207581). The cleavage occurs at the cell surface and probably in the serum of infected animals as well; both native and cleaved PA are able to bind to the cell receptor (PubMed:8051159, PubMed:11207581). The release of PA-20 from the remaining receptor-bound PA-63 exposes the binding site for EF and LF, and promotes oligomerization and internalization of the protein (PubMed:8051159, PubMed:11207581). Belongs to the bacterial binary toxin family.