E3 ubiquitin-protein ligase XIAP / BIRC4, also known as inhibitor of apoptosis protein 3, X-linked inhibitor of apoptosis protein, and IAP-like protein, is a protein that belongs to a family of apoptotic suppressor proteins. Members of this family share a conserved motif termed, baculovirus IAP repeat, which is necessary for their anti-apoptotic function. XIAP / BIRC4 functions through binding to tumor necrosis factor receptor-associated factors TRAF1 and TRAF2 and inhibits apoptosis induced by menadione, a potent inducer of free radicals, and interleukin 1-beta converting enzyme. XIAP / BIRC4 also inhibits at least two members of the caspase family of cell-death proteases, caspase-3 and caspase-7. Mutations in this encoding gene are the cause of X-linked lymphoproliferative syndrome. Alternate splicing results in multiple transcript variants. Thought to be the most potent apoptosis suppressor, XIAP / BIRC4, directly binds and inhibits caspases -3, -7 and -9. Survivin, which also binds to several caspases, is up-regulated in a many tumour cell types. Defects in XIAP / BIRC4 are the cause of lymphoproliferative syndrome X-linked type 2 (XLP2). XLP is a rare immunodeficiency characterized by extreme susceptibility to infection with Epstein-Barr virus (EBV). Symptoms include severe or fatal mononucleosis, acquired hypogammaglobulinemia, pancytopenia and malignant lymphoma.
XIAP (X-chromosome linked inhibitor of apoptosis) is a member of the apoptosis (IAP) family of proteins that inhibit caspases. The BIR2 domain of XIAP inhibits caspase-3 and caspase-7. The ability of XIAP to inhibit caspases is prevented by SMAC/Diablo through binding to XIAP-BIR2 and-BIR3 domains.
Aminopeptidase P (APP) is a hydrolase specific for N-terminal imido bonds; which are common to several collagen degradation products; neuropeptides; vasoactive peptides; and cytokines. A membrane-bound and soluble form of this enzyme (XPNPEP2) have been identified as products of two separate genes. XPNPEP2; the X-linked gene that encodes membranous aminopeptidase P (APP); has been reported to associate with APP activity. The membrane aminopeptidase P (XPNPEP2) is largely limited in distribution to endothelia and brush border epithelia. APP and XPNPEP2 contain homologous blocks of sequence common to members of the "pita bread-fold" protein family; of which Escherichia coli methionine aminopeptidase is the prototype. The C-2399A variant in XPNPEP2 is associated with reduced APP activity and a higher incidence of AE-ACEi. XPNPEP2 mRNA was detected in fibroblasts that carry the translocation; suggesting that this gene at least partially escapes X inactivation. XPNPEP2 is a candidate gene for premature ovarian failure (POF).
Xaa-Pro aminopeptidase 2 (XPNPEP2) belongs to the peptidase M24B family of metalloproteases. Human XPNPEP2 is widely expressed in various tissues; such as kidney; lung; heart; placenta; liver; small intestine and colon. However; it doesn’t express in brain; skeletal muscle; pancreas; spleen; thymus; prostate; testis and ovary. XPNPEP2 is a Homotrimer which binds 2 manganese ions per subunit. The metalloprotease XPNPEP2 may play a role in the inflammatory process and other reactions produced in response to injury or infection and the metabolism of the vasodilator bradykinin.
Probable Xaa-Pro Aminopeptidase 3 (XPNPEP3) is a member of the peptidase M24B family. XPNPEP3 has two isoforms and both are widely expressed. XPNPEP3 is localized in the Mitochondrion. XPNPEP3 catalyzes the release of any N-terminal amino acid, including proline, that is linked to proline, even from a dipeptide or tripeptide. Defects in XPNPEP3 are the cause of nephronophthisis-like nephropathy type 1 which is a disorder with features of nephronophthisis, a cystic kidney disease leading to end-stage renal failure.
X-ray repair cross-complementing protein 5, also known as 86 kDa subunit of Ku antigen, ATP-dependent DNA helicase 2 subunit 2, ATP-dependent DNA helicase II 80 kDa subunit, CTC box-binding factor 85 kDa subunit, DNA repair protein XRCC5, Lupus Ku autoantigen protein p86, TLAA and XRCC5, is a nucleus and chromosome which belongs to theku80 family. XRCC5 is a single stranded DNA-dependent ATP-dependent helicase. XRCC5 has a role in chromosome translocation. X-ray repair cross-complementing protein 6, also known as 5'-deoxyribose-5-phosphate lyase Ku70, ATP-dependent DNA helicase 2 subunit 1, ATP-dependent DNA helicase II 70 kDa subunit, 70 kDa subunit of Ku antigen, ATP-dependent DNA helicase 2 subunit 1, CTC box-binding factor 75 kDa subunit, Lupus Ku autoantigen protein p70, Thyroid-lupus autoantigen and XRCC6, is a nucleus and chromosome which belongs to theku70 family. Heterodimer of a XRCC6 and a XRCC5 subunit associates in a DNA-dependent manner with PRKDC to form the DNA-dependent protein kinase complex DNA-PK, and with the LIG4-XRCC4 complex. The dimer also associates with NAA15, and this complex binds to the osteocalcin promoter and activates osteocalcin expression.
X-Prolyl Aminopeptidase (XPNPEP1) is a proline-specific metalloaminopeptidase that specifically catalyzes the removal of any unsubstituted N-terminal amino acid that is adjacent to a penultimate proline residue. Because of its specificity toward proline, it has been suggested that X-Prolyl Aminopeptidase is important in the maturation and degradation of peptide hormones, neuropeptides, and tachykinins, as well as in the digestion of otherwise resistant dietary protein fragments, thereby complementing the pancreatic peptidases. X-Prolyl Aminopeptidase is a member of the M24 family of metalloproteases, which also contains methionine aminopeptidases, X-Pro dipeptidase, aminopeptidase P2, aminopeptidase P homolog, proliferation-associated protein 1, and suppressor of Ty homolog or chromatin-specific transcription elongation factor large subunit. It is a soluble enzyme, in contrast to the GPI-anchored Aminopeptidase P2 encoded by XPNPEP2. Deficiency of X-Prolyl Aminopeptidase results in excretion of large amounts of imino-oligopeptides in urine. Human Aminopeptidase P1 is widely expressed. The amino acid sequence of human X-Prolyl Aminopeptidase is 99%, 97%, 95%, 74% and 73% identical to that of canine, bovine, mouse/rat, Xenopus and zebrafish, respectively.
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