FIX is secreted

Stable Identifier
Reaction [uncertain]
Homo sapiens
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Coagulation Factor IX (FIX) is expressed by hepatocytes (Yoshitake et al. 1985; Kurachi K & Kurachi S 1995). The newly synthesised FIX protein molecule comprising a pre- and pro-sequence (28 and 18 amino acids, respectively) and a mature peptide of 415 amino acids (total length, 461 amino acids) (Yoshitake et al. 1985; Kurachi K & Kurachi S 1995; Andersson LO et al. 1975; Anson DS et al. 1984). The pre-sequence (or signal sequence) directs FIX for secretion and the pro-sequence provides a binding domain for a vitamin K dependent (VKD) gamma (γ)-glutamyl carboxylase (GGCX) (Fryklund L et al. 1976; Galeffi P & Brownlee GG 1987; Lingenfelter SE & Berkner K 1996; Stanley TB et al. 1998). GGCX, an integral membrane protein located in the endoplasmic reticulum (ER) of hepatocytes, carboxylates certain glutamic acid residues in the adjacent GLA domain of FIX (Presnell and Stafford, 2002; Fryklund L et al. 1976; Galeffi P & Brownlee GG 1987). During the γ-carboxylation process, vitamin K hydroquinone is oxidized to vitamin K 2,3 epoxide and a carboxyl group is added to a glutamic acid residue (Wallin R eet al. 2002). In its native form, FIX contains 12 glutamic acid residues in the Gla domain; the first 10 residues are conserved in all VKD proteins, whereas the last two are unique to FIX (Gillis et al. 2008). FIX undergoes several other post-translational modifications before its secretion, including N- and O-linked glycosylation, sulfation, phosphorylation and hydroxylation (Agarwala KL et al. 1994; Bharadwaj D et al. 1995; Kaufman RJ 1998; Bond M et al. 1998; Enjolras N et al. 2004). These post-translational modifications occur within the ER and Golgi apparatus. In the ER, maturation and processing of secreted proteins are orchestrated by a group of molecules which facilitate protein folding and ensure that only correctly folded, assembled and modified proteins are transported along the secretory pathway. The proteins involved in the folding system are lectins such as calreticulin (CRT) or calnexin (CNX). A cellular unfolded protein response induces the ER-resident molecular chaperones such as glucose-regulated protein GRP78/BiP to prevent the aggregation of proteins in the ER. FIX was shown to co-immunoprecipitate with GRP78/BiP and CRT In cell lysates of transiently transfected human hepatocellular carcinoma (HepG2) cells expressing FIX (Enjolras N et al. 2004). After transportation of the carboxylated pro-FIX into the Golgi apparatus, the propeptide (29-46) is removed by the paired basic amino acid cleaving enzyme (PACE) (Wasley LC et al. 1993). The removal of the propeptide by PACE influences the formation of Ca2+-induced secondary and tertiary structures of the Gla domain, thus it is required for normal function of FIX (Pipe, 2008). The mature FIX is secreted and circulates in the plasma as an inactive 57kDa zymogen form (47-461). Domains within the zymogen are identified according to structure or function as follows: the GLA domain is crucial for the interaction with phospholipid surfaces; two epidermal growth factor (EGF)-like domains are critical for the interactions between factor IX and factor VIIIa; the activation peptide is released after proteolytic activation and the catalytic serine protease domain is required for normal function of FIX (Pipe 2008; Yoshitake S et al. 1985; Di Scipio RG et al. 1977; Rees DJ et al. 1988; Freedman SJ et al. 1995). Activation of factor IX involves cleavage of two peptide bonds, one on the C-terminal side of arginine 191 (the α-cleavage) the other on the C-terminal side of arginine 226 (the β-cleavage) (Di Scipio RG et al. 1978; Zögg T & Brandstetter H 2009). Activated factor IX comprising an N-terminal light chain and a C-terminal heavy chain held together by a disulphide bridge between cysteine resides 178 and 335 (Di Scipio RG et al. 1978; Zögg T & Brandstetter H 2009).
Literature References
PubMed ID Title Journal Year
8148489 Biology of factor IX

Furukawa, M, Kurachi, K, Yao, SN, Kurachi, S

Blood Coagul. Fibrinolysis 1993
3733688 Expression, purification, and characterization of recombinant gamma-carboxylated factor IX synthesized in Chinese hamster ovary cells

Kaufman, RJ, Furie, BC, Shoemaker, CB, Furie, B, Wasley, LC

J. Biol. Chem. 1986
Orthologous Events
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