Calreticulin is a candidate for a calsequestrin-like function in Ca2+-storage compartments (calciosomes) of liver and brain

Abstract

In a search for the non-muscle equivalent of calsequestrin (the low-affinity high-capacity Ca2(+)-binding protein responsible for Ca2+ storage within the terminal cisternae of the sarcoplasmic reticulum), acidic proteins were extracted from rat liver and brain microsomal preparations and purified by column chromatography. No calsequestrin was observed in these extracts, but the N-terminal amino acid sequence of the major Ca2(+)-binding protein of the liver microsomal fraction was determined and found to correspond to that of calreticulin. This protein was found to bind approx. 50 mol of Ca2+/mol of protein, with low affinity (average Kd approx. 1.0 mM). A monoclonal antibody, C6, raised against skeletal-muscle calsequestrin cross-reacted with calreticulin in SDS/PAGE immunoblots, but polyclonal antibodies reacted with native calreticulin only weakly, or not at all, after SDS denaturation. Immuno-gold decoration of liver ultrathin cryosections with affinity-purified antibodies against liver calreticulin revealed luminal labelling of vacuolar profiles indistinguishable from calciosomes, the subcellular structures previously identified by the use of anti-calsequestrin antibodies. We conclude that calreticulin is the Ca2(+)-binding protein segregated within the calciosome lumen, previously described as being calsequestrin-like. Because of its properties and intraluminal location, calreticulin might play a critical role in Ca2+ storage and release in non-muscle cells, similar to that played by calsequestrin in the muscle sarcoplasmic reticulum.

Purkinje cells of rat and chicken cerebellum contain calreticulin (CaBP3)

“…This protein has an apparent molecular mass of 60 kDa on SDS-PAGE, and a molecular mass of 46 kDa has been established by molecular cloning [3,4]. Calreticulin is found mainly in the ER [3] and in vesicles called calciosomes ( [2], see [5] as a review). Its function remains unclear although a role in calcium sequestration has been proposed.…”

Structural and functional aspects of calcium homeostasis in eukaryotic cells

“…However MacLennan and coworkers, who first described calreticulin [223], had already reported its capacity to bind more than 20 mol Ca2+/mol protein with low affinity. We have recently characterized calreticulin from rat liver and observed that (in addition to the high-affinity site) it also binds up to 50 mol Ca2+/mol protein with low affinity, (Kd = 1 mM) [299]. Calreticulin cross-reacts with a few anti-(skeletal muscle calsequestrin) antibodies, particularly in its native form [299].…”

“…We have recently characterized calreticulin from rat liver and observed that (in addition to the high-affinity site) it also binds up to 50 mol Ca2+/mol protein with low affinity, (Kd = 1 mM) [299]. Calreticulin cross-reacts with a few anti-(skeletal muscle calsequestrin) antibodies, particularly in its native form [299]. In ultrathin cryosections, anti-calreticulin antibodies, coated with gold particles, localize in vesicular structures which are morphologically indistinguishable from calciosomes [299].…”

“…Calreticulin cross-reacts with a few anti-(skeletal muscle calsequestrin) antibodies, particularly in its native form [299]. In ultrathin cryosections, anti-calreticulin antibodies, coated with gold particles, localize in vesicular structures which are morphologically indistinguishable from calciosomes [299]. Calreticulin is a glycoprotein (molecular mass from cDNA 47 kDa); the polysaccharide side chain is of the Golgi type and contains a terminal galactose suggesting that the protein has reached the trans Golgi network [304].…”

Intracellular Calreticulin Regulates Multiple Steps in Fibrillar Collagen Expression, Trafficking, and Processing into the Extracellular Matrix

“…It also acts as a classical chaperone for nonglycosylated proteins by preventing protein aggregation in an in vitro model (22). CRT is an important regulator of Ca 2ϩ homeostasis within the ER (23,24). Total CRT expression is up-regulated by many forms of cellular stress, including amino acid deprivation, depletion of Ca 2ϩ stores, oxidative stress, and hypoxia (25)(26)(27)(28)(29).…”