4 Phosphorylation of BiP occurred in a highly conserved region of the peptide-binding domain, 4 which plays an important role in the regulation of the chaperone activity of BiP. We have recently shown that inhibition of TOR signaling by rapamycin in Chlamydomonas led to the phosphorylation of BiP on threonine residues, suggesting a role of TOR in the control of BiP modification. Whether regulation of BiP function by phosphorylation and/or ADP-ribosylation is conserved in other systems is unclear. 3 According to this theory, unmodified BiP monomers are more active and bind to and promote folding of substrate proteins. 2 Mammalian BiP can be post-translationally modified by phosphorylation and ADP-ribosylation and both modifications have been associated to oligomeric forms of the protein that likely represent an inactive state of BiP. 1 This chaperone assists the folding and assembly of newly-synthesized proteins as they are translocated into the ER and also binds to misfolded, underglycosylated or unassembled proteins.
BiP is a member of the HSP70 family of molecular chaperones that resides within the lumen of the ER.
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