Calreticulin Is a receptor for nuclear export.
In previous work, we used a permeabilized cell assay that reconstitutes nuclear export of protein kinase inhibitor (PKI) to show that cytosol contains an export activity that is distinct from Crm1 (Holaska, J.M., and B.M. Paschal. 1995. Proc. Natl. Acad. Sci. USA. 95: 14739-14744). Here, we describe the purification and characterization of the activity as calreticulin (CRT), a protein previously ascribed to functions in the lumen of the ER. We show that cells contain both ER and cytosolic pools of CRT. The mechanism of CRT-dependent export of PKI requires a functional nuclear export signal (NES) in PKI and involves formation of an export complex that contains RanGTP. Previous studies linking CRT to downregulation of steroid hormone receptor function led us to examine its potential role in nuclear export of the glucocorticoid receptor (GR). We found that CRT mediates nuclear export of GR in permeabilized cell, microinjection, and transfection assays. GR export is insensitive to the Crm1 inhibitor leptomycin B in vivo, and it does not rely on a leucine-rich NES. Rather, GR export is facilitated by its DNA-binding domain, which is shown to function as an NES when transplanted to a green fluorescent protein reporter. CRT defines a new export pathway that may regulate the transcriptional activity of steroid hormone receptors.
Holaska
J M
JM
Center for Cell Signaling, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA.
Black
B E
BE
Love
D C
DC
Hanover
J A
JA
Leszyk
J
J
Paschal
B M
BM
D021581:Active Transport, Cell Nucleus; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D037282:Calreticulin; D002352:Carrier Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D003600:Cytosol / Q000378:metabolism; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D011965:Receptors, Glucocorticoid / Q000378:metabolism; D012261:Ribonucleoproteins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism
D002135:Calcium-Binding Proteins; D037282:Calreticulin; D002352:Carrier Proteins; D005231:Fatty Acids, Unsaturated; D047908:Intracellular Signaling Peptides and Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D011965:Receptors, Glucocorticoid; D012261:Ribonucleoproteins; C015482:protein kinase modulator; D017868:Cyclic AMP-Dependent Protein Kinases; D020931:ran GTP-Binding Protein; C038753:leptomycin B