protein disulfide isomerase (cellular thyroid hormone binding protein, prolyl 4-hydroxylase subunit beta, P4HB, ERBA2L, PDI, PDIA1, PO4DB)

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Function

multifunctional protein
catalyzes formation, breakage & rearrangement of disulfide bonds
at the cell surface, seems to act as a reductase that cleaves disulfide bonds of proteins attached to the cell; may cause structural modifications of exofacial proteins
inside the cell, seems to form/rearrange disulfide bonds of nascent proteins
at high concentrations, functions as a chaperone that inhibits aggregation of misfolded proteins
at low concentrations, facilitates aggregation (anti-chaperone activity)
may be involved with other chaperones in the structural modification of the trans-Golg precursor in hormone biogenesis
acts a structural subunit of:
- prolyl 4-hydroxylase
- microsomal triacylglycerol transfer protein MTTP stabilizes both enzymes & retains them in the ER without contributing to the catalytic activity
catalyzes the rearrangement of -S-S- bonds in proteins
binds UBQLN1
binds to CD4, & upon HIV-1 binding to the cell membrane, is part of a P4HB/PDI-CD4-CXCR4-gp120 complex

endoplasmic reticulum lumen, melanosome
in some cell types, seems to be also secreted or associated with the plasma membrane, where it undergoes constant shedding & replacement from intracellular sources (probable
localizes near CD4-enriched regions on lymphoid cell surfaces

reduces & may activate fusogenic properties of HIV-1 gp120 surface protein, thereby enabling HIV-1 entry into the cell