Beta-2 microglobulin

β₂ microglobulin also known as B2M is a component of MHC class I molecules, which are present on all nucleated cells (excludes red blood cells).^[1][2] In humans, the β₂ microglobulin protein^[3] is encoded by the B2M gene.^[2][4]

Structure and function

β₂ microglobulin lies lateral to the α₃ chain on the cell surface. Unlike α₃, β₂ has no transmembrane region. Directly above β₂ (i.e. away from the cell) lies the α₁ chain, which itself is lateral to the α₂.

β₂ microglobulin associates not only with the alpha chain of MHC class I molecules, but also with class I-like molecules such as CD1 and Qa.

An additional function is association with the HFE protein, together regulating endocytosis of iron into intestinal cells. Loss of this function causes iron excess and hemochromatosis.

Clinical significance

In patients on long-term hemodialysis, it can aggregate into amyloid fibers that deposit in joint spaces, a disease known as dialysis-related amyloidosis.

Mice models deficient for the β₂ microglobulin gene have been engineered. These mice demonstrate that β₂ microglobulin is necessary for cell surface expression of MHC class I and stability of the peptide binding groove. In fact, in the absence of β₂ microglobulin, very limited amounts of MHC class I (classical and non-classical) molecules can be detected on the surface. In the absence of MHC class I, CD8 T cells cannot develop. (CD8 T cells are a subset of T cells involved in the development of acquired immunity.)Low levels of β₂ microglobulin can indicate non-progression of HIV.

Levels of beta-2 microglobulin can be elevated in multiple myeloma and lymphoma,^[5] though in these cases primary amyloidosis (amyloid light chain) and secondary amyloidosis (Amyloid associated protein) are more common. the normal value of beta-2 microglobulin ranges from( 1 - 2.1 )

References

Further reading

External links

• MeSH beta+2-Microglobulin