title: Regulation of protein compartmentalization expands the diversity of protein function.
abstract: Proteins destined for the secretory pathway are translocated into the endoplasmic reticulum (ER) by signal sequences that vary widely in their functional properties. We have investigated whether differences in signal sequence function have been exploited for cellular benefit. A cytosolic form of the ER chaperone calreticulin was found to arise by an aborted translocation mechanism dependent on its signal sequence and factors in the ER lumen and membrane. A signal sequence that functions independently of these accessory translocation factors selectively eliminated cytosolic calreticulin. In vivo replacement of endogenous calreticulin with a constitutively translocated form influenced glucocorticoid receptor-mediated gene activation without compromising chaperone activity in the ER. Thus, in addition to its well-established ER lumenal functions, calreticulin has an independent role in the cytosol that depends critically on its inefficient compartmentalization. We propose that regulation of protein translocation represents a potentially general mechanism for generating diversity of protein function.
authors.affiliation: Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Kelly L
authors.initials: KL
authors.lastname: Shaffer
authors.forename: Ajay
authors.initials: A
authors.lastname: Sharma
authors.forename: Erik L
authors.initials: EL
authors.lastname: Snapp
authors.forename: Ramanujan S
authors.initials: RS
authors.lastname: Hegde
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D037282:Calreticulin / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005786:Gene Expression Regulation; D006801:Humans; D008969:Molecular Sequence Data; D011388:Prolactin / Q000235:genetics / Q000378:metabolism; D021382:Protein Sorting Signals; D051381:Rats; D011965:Receptors, Glucocorticoid / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: D037282:Calreticulin; D021382:Protein Sorting Signals; D011965:Receptors, Glucocorticoid; D011993:Recombinant Fusion Proteins; D011388:Prolactin