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Stress Sensing in Plants by an ER Stress Sensor/Transducer, bZIP28


Stress Sensing in Plants by an ER Stress Sensor/Transducer, bZIP28

Renu Srivastava et al. Front Plant Sci.


Two classes of ER stress sensors are known in plants, membrane-associated basic leucine zipper (bZIP) transcription factors and RNA splicing factors. ER stress occurs under adverse environmental conditions and results from the accumulation of misfolded or unfolded proteins in the ER lumen. One of the membrane-associated transcription factors activated by heat and ER stress agents is bZIP28. In its inactive form, bZIP28 is a type II membrane protein with a single pass transmembrane domain, residing in the ER. bZIP28's N-terminus, containing a transcriptional activation domain, is oriented towards the cytoplasm and its C-terminal tail is inserted into the ER lumen. In response to stress, bZIP28 exits the ER and moves to the Golgi where it is proteolytically processed, liberating its cytosolic component which relocates to the nucleus to upregulate stress-response genes. bZIP28 is thought to sense stress through its interaction with the major ER chaperone, binding immunoglobulin protein (BIP). Under unstressed conditions, BIP binds to intrinsically disordered regions in bZIP28's lumen-facing tail and retains it in the ER. A truncated form of bZIP28, without its C-terminal tail is not retained in the ER but migrates constitutively to the nucleus. Upon stress, BIP releases bZIP28 allowing it to exit the ER. One model to account for the release of bZIP28 by BIP is that BIP is competed away from bZIP28 by the accumulation of misfolded proteins in the ER. However, other forces such as changes in energy charge levels, redox conditions or interaction with DNAJ proteins may also promote release of bZIP28 from BIP. Movement of bZIP28 from the ER to the Golgi is assisted by the interaction of elements of the COPII machinery with the cytoplasmic domain of bZIP28. Thus, the mobilization of bZIP28 in response to stress involves the dissociation of factors that retain it in the ER and the association of factors that mediate its further organelle-to-organelle movement.

Keywords: COPII vesicle transport system; Golgi apparatus; bZIP transcription factors; binding immunoglobulin protein (BIP); endoplasmic reticulum stress; protein folding; unfolded protein response (UPR).


Mobilization of bZIP28 in response to stress. Under unstressed conditions, bZIP28 resides in the ER membrane and is thought to be tethered there by the interaction of its lumenal tail with Binding Protein (BIP). In response to adverse environmental conditions or to an overload in the protein synthesis, unfolded proteins accumulate in the ER and BIP is competed away from its binding to bZIP28. Once freed, bZIP28 interacts with Sar1 GTPase a component of the COPII transport system and transported to the Golgi apparatus. In the Golgi, bZIP28 is processed by resident proteases including Site-2-Protease (S2P) liberating bZIP28’s cytoplasmic domain, which relocates in the nucleus where it upregulates stress response genes. (Figure based on Srivastava et al., 2013).
Map (above) of Arabidopsis bZIP28 showing its cytosolic, transmembrane (TMD) and lumenal domains. Also shown are the locations of the bZIP region in the cytosolic domain, the canonical S1P recognition site (RRIL) in the lumenal domain, several alanine substitution mutations and a deletion mutation (bZIP28Δ355) described in the text. Amino acid sequence of bZIP28 (below) highlights the bZIP domain and S1P recognition site (both underlined), the TMD in which the VASI sequence and helix-breaking G residue are boxed. Paired lysines involved in Sar1b binding and Golgi relocalization are indicated in bold. (Figure based on Srivastava et al., 2012).
Ribbon structure of the bZIP28 lumenal domain predicted by the template-based prediction program, I-TASSER (Zhang, 2008). The regions of the protein highlighted with the white dashed lines represent the peptides to which BIP preferentially binds in a phage display assay (Srivastava et al., 2013).

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