GRP78: Molecular Weight
Immunofluorescent detection of GRP78 in mouse fibroblast cells using using Anti-GRP78 (6H4-2G7
Grp78/BiP (HspA5, Hsp70-5) is a member of the 70 kDa heat shock protein (HSPA5) family of molecular chaperones 1, 2
. The HSPA family represents the most conserved and best characterized group of HSPs comprising polypeptides whose molecular weights range from 62 to 78 kDa and that are encoded by a multigene family encompassing up to 17 genes and 30 pseudogenes in humans 50
. Functional genes encoding HSP70 proteins
map to several human chromosomes as given in Table 2 51
. Grp78/BiP is encoded by a single-copy nuclear gene clustered on chromosome 9q33.3 and translated in the cytosol. Two pseudogenes have been found as being associated with this gene, termed LOC400750
and located to chromosome 1 and 4, respectively. The corresponding protein consists of 654 amino acid residues and harbors an ER-retention signal sequence of four amino acids (KDEL) at the C-terminus. Cleavage of the signal peptide at the N-terminus generates the mature Grp78/BiP protein with 636 amino acids and a molecular mass of 72.3 kDa which is further processed by co-/post-translational modifications yielding a molecular weight of 78 kDa. As demonstrated previously, mammalian Grp78/BiP not complexed to protein ligands can form homo-dimeric complexes that are subjected to post-translational modifications 49
. Moreover, Grp78/BiP-IgG heavy-chain complexes of apparently 350 kDa could be detected in the same study. The studies by Toledo et al.
identified Grp78-IgE Fc complexes ranging from 100 kDa to 300 kDa in size 52
. The most stable Grp78/BiP-IgE Fc complex is thought to consist of a Grp78/BiP dimer associated with an IgE-Fc dimer yielding a complex close to 300 kDa (higher oligomeric complexes are also possible) 52
Recently, a novel cytosolic isoform of Grp78/BiP was identified in mouse and humans termed Grp78va 53. Grp78va is produced by alternative splicing of the corresponding HSPA5 pre-mRNA and alternative translation initiation, yielding a putative Grp78va protein of 507 aa and a molecular mass of ~62 kDa. Grp78va obviously harbors a truncated N-terminus and is devoid of the N-terminal ER targeting signal 53.