Calreticulin is the major calcium binding protein
found in smooth muscle sarcoplasmic reticulum (SR) and non-muscle
endoplasmic reticulum (ER) membranes. This protein was originally
identified in SR membranes and plays a minor role in calcium storage
in skeletal and cardiac muscle SR. Calreticulin is also known as
calregulin, CRP55, CaBP3, calsequestrin-like protein and Ro/SS-A
antigen. Calreticulin binds calcium with low affinity and high
capacity, however it also exhibits a single high affinity binding
site. The highly conserved sequence Lys-Asp-Glu-Leu (KDEL) is present
at the C-terminus of calreticulin and other resident ER proteins
including glucose regulated protein 78 (GRP78), GRP94 and protein
disulfide isomerase (PDI). This sequence is responsible for the
retention of newly synthesized proteins within the ER lumen. This
retention is reported to be mediated by a KDEL receptor. Recent
reports indicate that calreticulin can act as a modulator of the
regulation of gene transcription by nuclear hormone receptors and
may also act as a molecular chaperone.
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