Protein bodies: how the ER deals with high accumulation of recombinant proteins.

Reza Saberianfar,Rima Menassa

doi:10.1111/pbi.12730

Reza Saberianfar, Rima Menassa

Open Access

https://doi.org/10.1111/pbi.12730

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Abstract

Protein bodies (PBs) are highly specialized protein storage organelles in cereal seeds. PB formation in seeds initiates in the endoplasmic reticulum (ER), and depending on the plant species, PBs remain in the ER or find their way out of the ER, bypass the Golgi and end up in protein storage vacuoles (PSVs) (Khan et al., 2012). Protein bodies have been ectopically induced in leaves by producing high amounts of ER-retrieved recombinant proteins, usually by fusing them to protein tags such as Zera®, elastin-like polypeptide (ELP) and hydrophobin-I (HFBI). This article is protected by copyright. All rights reserved.

Highlights

Protein bodies have been ectopically induced in leaves by producing high amounts of endoplasmic reticulum (ER)-retrieved recombinant proteins, usually by fusing them to protein tags such as Zeraâ, elastin-like polypeptide (ELP) and hydrophobin-I (HFBI)
Protein bodies (PBs) induced by these processes are numerous, round and clustered, they remain in the ER, and they are not destined to the central vacuole (Llop-Tous et al, 2010; Saberianfar et al, 2015, 2016)
This strategy, does not work as well with Zeraâ, because the strong affinity of Zeraâ molecules to one another results in a condensed and sticklike alignment of Zeraâ molecules which excludes other molecules from the core of PBs (Llop-Tous et al, 2010). This was confirmed with co-expression of secretory GFP or ER-targeted GFP with Zeraâ-DsRed in which the GFP signal localized to the periphery of Zeraâ PBs and not in their core, and explains why Zeraâ-DsRed has no significant effect on EPO accumulation (Saberianfar et al, 2016)

Summary

PB formation initiates upon reaching a threshold level

The simultaneous monitoring of recombinant protein accumulation and PB formation suggested that a recombinant protein threshold of 0.2% total soluble protein is required for PB formation (Figure 1A). Such proteins are likely sequestered in PBs where they are prevented from progressing through the secretory pathway, and over time, their concentration in PBs increases (Figure 1B) This PB-trapping strategy is important for two reasons; it helps to increase accumulation levels of difficultto-express recombinant proteins and eliminates the need for the addition of fusion tags which might affect the proper folding and activity of the protein of interest. This strategy, does not work as well with Zeraâ, because the strong affinity of Zeraâ molecules to one another results in a condensed and sticklike alignment of Zeraâ molecules which excludes other molecules from the core of PBs (Llop-Tous et al, 2010). This was confirmed with co-expression of secretory GFP or ER-targeted GFP with Zeraâ-DsRed in which the GFP signal localized to the periphery of Zeraâ PBs and not in their core, and explains why Zeraâ-DsRed has no significant effect on EPO accumulation (Saberianfar et al, 2016)

ER is the initiation point and the final destination of PBs

Future prospects