translated, generating Haclp, a basic leucine zipper (bZIP) transcription factor that
activates the transcription of ER chaperone genes by binding an unfolded protein
response element (UPRE) in their promoters.
The machinery of the mammalian UPR is somewhat more complex and involves
several signal transduction pathways (Ma and Hendershot, 2001). Two mammalian
homologs of Ire1p have been identified, Ire1α and Irelβ (Tirasophon et al., 1998;
Wang et al., 1998a). Ire1α is ubiquitously expressed, while expression of Ire1β is
restricted to the epithelium of the gut. Both proteins have conserved kinase and
endoribonuclease domains and appear to function similarly to yeast Irep1. A wealth
of recent studies suggests a model whereby under normal conditions the luminal
domain of Irel is stably associated with the ER chaperone, BiP, but, following ER
stress, BiP dissociates from Irel to bind unfolded proteins (Patil and Walter, 2001).
Free Irel then oligomerizes and undergoes trans-autophosphorylation, and the
activated endonuclease domain excises a short sequence (26 bp in man) from the
mRNA of the X-Box binding protein (XBP-1), a bZIP transcription factor (Ma and
Hendershot, 2001; Shen et al., 2001; Yoshida et al., 2001; Calfon et al., 2002). This
switches the reading frame of XBP-1 mRNA and generates a new protein encoding
the original N-terminal DNA binding domain and a new C-terminal transactivation
domain that binds and promotes transcription at UPREs in target genes. The UPR
in mammals also involves ATF6, an ER transmembrane protein with a cytosolic bZIP
transcription factor domain that is cleaved off and released to the nucleus by S1P and
S2P proteases following ER Stress (Ye et al., 2000). Like XBP-1, cleaved ATF6 binds
UPREs and upregulates ER chaperones (Yoshida et al., 1998a). ATF6 has also been
shown to upregulate XBP-1 mRNA, presumably enhancing Irel directed cleavage
(Yoshida et al., 2001). ATF6 and Irel seem to represent redundant signaling pathways
since Ire1α-/- and Ire1α-/-, Irelβ-/- MEFs show no defect in the UPR induction of
chaperone genes, although the Ire1α-/- phenotype confers embryonic lethality,
indicating the importance of this pathway in development (Urano et al., 2000a).
The mammalian UPR has also evolved a third signaling branch that rapidly shuts
down translation after an insult, preventing the continual accumulation of newly
synthesized proteins into the ER when protein-folding conditions are compromised
(Kaufman, 1999). This is achieved through a third transmembrane kinase, PERK
(Harding et al., 1999). The luminal domain of PERK is homologous to Ire1 and
probably senses unfolded proteins in a similar manner. However, the cytosolic tail of
PERK encodes a kinase domain that phosphorylates the translation initiation factor
eIF2α, preventing the assembly of the 80s ribosomal initiation complex and halting
general translation (Harding et al., 1999), while selectively allowing the upregulation
of ER chaperones and regulatory components of the secretory pathway (Harding et
al., 2000a; Niwa and Walter, 2000). PERK signaling also downregulates cyclin D
levels, resulting in cell-cycle arrest in the G1 phase (Brewer and Diehl, 2000), which
may provide time for the cell to restore balance in the ER (Niwa and Walter, 2000).
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