Secretion chaperones

In addition to its role as the P-subunit of PHY, PDI acts independently by catalysing thiol/protein disulphide interchange. The role of PDI as the P-subunit in prolyl 4-hydroxylase is not related to its disulphide isomerase activity and experiments where the vertebrate PDI was mutated in both thioredoxin-like active domains had no effect on tetramer assembly (Vuori et al., 1992). PDI appears to function as a molecular chaperone, retaining the a-subunits in the correct catalytically active, non-aggregated form in the ER-lumen (John et al, 1993). Dissociation of the P-subunits results in insoluble aggregates of the a-subunits, analogous to a-subunits expressed in the absence of PDI. An additional function of PDI in the complex is to maintain the ER luminal location of the a-subunits, since deletion of the ER retention signal from PDI results in the secretion of the complex (Vuori et al., 1992). 

Delepelaire, P., and Wandersman C. (1998). The SecB chaperone is involved in the secretion of the Serratia marcescens hasa protein through an ABC transporter. EMBO J. 17, 936-944. 

Unlike transport across the membranes of the ER, transport across plasma membranes of bacteria often requires both hydrolysis of ATP and energy provided by the membrane electrical potential.33 38 44-48 Secretion into the periplasmic space has been well characterized but less is known about transport of proteins into the external membranes of E. coli48 A16 kDa periplasmic chaperone may be required.483... 

SERCA pumps sequester Ca2+ in the ER lumen By maintaining appropriate Ca2+ concentrations in the ER lumen, SERCA pumps also play an essential role in protein synthesis, folding and transport of membrane and secreted proteins. This involves in particular chaperone-dependent processing and post-translational modifications which require a unique calcium rich environment. Chaperone molecules such as calreticulin and calnexin are involved in the quality control pathway in the ER (Berridge, 2002 Ellgaard and Helenius, 2003 Michalak et al., 2002). 

Hsu, T. A. and Betenbaugh, M. J. (1997) Coexpression of molecular chaperone BiP improves immunoglobulin solubility and IgG secretion from Trichoplusia ni insect cells. Biotechnol. Prog. 13, 96-104. 

The core filament of B. mori silk, fibroin, is composed of a heavy chain fibroin (H-fibroin, 391 kDa) and a light chain fibroin (L-fibroin, 28 kDa), as well as P25 protein (25 kDa). These three constituents assemble into the secretory units in the ratio of 6 6 1 (H L P25) (Inoue et al., 2000 Shimura et al., 1976). H-fibroin and L-fibroin link together via a disulfide bond and the P25 is thought to act as a kind of chaperon to assist the transport and secretion of the insoluble H-fibroin (Sehnal and Zurovec, 2004 Tanaka et al., 1999 Zhou et al., 2000). H-fibroin has a much higher molecular weight and takes up 90% weight of the core filament. The properties of the core filament are mainly attributed to H-fibroin, which is often referred to as fibroin. 

Punt PJ, van Gemeren IA, Drint-Kuijvenhoven J et al (1998) Analysis of the role of the gene bipA, encoding the major endoplasmic reticulum chaperone protein in the secretion of homologous and heterologous proteins in black Aspergilli. Appl Microbiol Biotechnol 50 447 154... 

The vector for protein expression in the periplasmic space of E. coli (3) (Fig. 1) contains a lac promotor, which can be induced with IPTG (Isopropyl-D-thiogalacto-pyranoside). Translation starts with the OmpA signal sequence of the outer membrane protein A of E. coli, followed by the chemokine cDNA. The signal sequence leads to protein secretion into the periplasm, where it is cleaved off by bacterial enzymes. The periplasmic space has an oxidizing milieu, which enables disulfide bond formation and contains molecular chaperones, which inhibit aggregation and support correct folding of proteins (8). The lysis of the periplasmic space is performed by four freeze/thaw cycles,... 

Fig. 5. Copper homeostasis in Enterococcus hirae. Under copper-limiting conditions, copper is pumped into the cell by CopA. The CopZ copper chaperone picks up copper at this site of entry. Under physiological copper conditions, Zn(II)CopY binds to the promoter and represses transcription of the cop operon. Under conditions of copper excess, Cu-CopZ donates Cu(I) to CopY, which leads to the replacement of the Zn(II), loss of DNA-binding affinity, and ultimately synthesis of the operon products. Excess copper is secreted by the CopB efflux pump. The substrate for this pump may be a copper-glutathione (GSH) complex, rather than Cu-CopZ.

SecB A MOLECULAR CHAPERONE OF Escherichia coil PROTEIN SECRETION PATHWAY... 

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