Retention signal

Once the proteins have passed the quality control system of the early secretory pathway, they are transported in vesicles via the individual compartments of the Golgi apparatus to the plasma membrane. Soluble proteins are transported in the vesicle lumen, membrane proteins are integrated in the vesicle membrane. The transport to the cell surface is the default pathway for secretory and membrane proteins. Proteins may also become part of one of the intracellular compartments along the secretory pathway, but only if they contain specific retention signals. 

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). 

I I Synthetic spider silk protein SOI I I ER retention signal (KDEL)... 

Tab. 15.1 Therapeutic antibodies produced in transgenic plants. See Fig. 15.1 for structures of the different antibodies and antibody fragments presented in the second column. Abbreviations TSP, total soluble protein FLW, fresh leafweight MSP, murine signal peptide SP, signal peptide KDEL, ER retention signal. In the column Expression + and - means respectively that the antibody was or was not expressed. 

Fig. 15.6 Glycosylation of antibodies produced with ER retention signals in tobacco plants. Antibodies fused with two (left panel) or four (middle panel) KDEL-ER retention signals are respectively or exclusively glycosylated with high mannose type, and probably non-immuno-genic, N-glycans.

PDI contains a C-terminal tetrapeptide sequence known as the endoplasmic retention signal, KDEL. This anchor mediates the interaction between plasma membrane and membranes of the Golgi apparatus via a KDEL receptor. The PDI KDEL receptor complex is recycled back into the endoplasmic reticulum (Xiao et al., 1999). It is thought that a saturation of the retention mechanism results in the secretion of PDI which is deposited on the cell membrane and stabilized by electrostatic interactions (Terada et al, 1995). The secreted PDI is termed cell surface PDI (csPDI)... 

Because proteins are synthesized within the cytosol (including on the surface of the ER), it is plausible to postulate that the localization at the cytosol is a default. Such a notion is challenged by the discoveries of several cytoplasmic targeting/retention signals. In one example, the N-terminal 42 residue segment of cyclin B directs it to the cytosol if it is deleted, it is transported to the nucleus and the addition of the segment to cyclin A causes its cytosolic retention (Pines and Hunter,... 

The most common antibody fragment expressed in plants is svFv. These antibody fragments are fused to the ER retention signal KDEL to increase protein yield. The major factor that limits accumulation of scFv in plant tissue is protein stability. This can be mitigated to a degree by intracellular targeting. 

Retention times within the cell may be increased through the use of nuclear retention signal peptides and this with other approaches are certain to be tried in the immediate future. 

Resurrection plant 168 Retention signal 521 Retinoblastoma protein (Rb) 574 Retinol-binding protein 58 Retrieval signal KDEL 521 Retroviruses 248... 

Bichet, D., Cornet, V., Ceib, S., Carlier, E., Volsen, S., Hoshi, T., Mori, Y. and De Waard, M. (2000) The l-ll loop of the Ca2+ channel a, subunit contains an endoplasmic reticulum retention signal antagonized by the beta subunit. Neuron 25, 1 77-190. 

Although previous studies suggested that SG is highly glycosylated, it appears to contain very little carbohydrate, since its expected Mr of 63 kDa is very close to the size obtained by SDS PAGE.36, 39 In addition, the enzyme appears to contain a C-terminal sequence KKXKX that is a putative retention signal for type I transmembrane ER proteins. However additional detailed studies must be done to provide evidence for the localization of SG in relation to the site of synthesis of strictosidine. 

Pines J, Hunter T. 1994. The differential localization of human cyclins A and B is due to a cytoplasmic retention signal in cyclin B. EMBO J 13 3772-3781. 

Scott DB, Blanpied TA, Swanson GT, Zhang C, Ehlers MD. 2001. An NMDA receptor ER retention signal regulated by phosphorylation and alternative splicing. J Neurosci 21 3063-3072. 

PDZ domain suppression of an ER retention signal in NMDA receptor NR1 splice variants. Neuron 28 887-898. 

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