The polymeric immunoglobulin receptor is the major calmodulin-binding protein in an endosome fraction from rat liver enriched in recycling receptors
✍ Scribed by C Enrich; S Jäckle; R J Havel
- Book ID
- 102851290
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 551 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0270-9139
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✦ Synopsis
In earlier research three endosomal fractions from livers of Rat liver endosomes contain one major high-affinity estradiol-treated rats, in which low-density lipoprotein (LDL) calmodulin-binding protein (CaMBP) that now has been receptors are expressed at high levels, were isolated and kiidentified as the polymeric immunoglobulin receptor netically and morphologically characterized. Endosome frac-(pIgR). In isolated endosomes pIgR was enriched in the tions from normal rats have similar ultrastructural and bioreceptor-recycling compartment (RRC); lesser enrichchemical characteristics. 6 ment was found in ''early'' endosome (CURL) and much-
To understand the properties and functions of the endocytic less in ''late'' endosome fractions (multivesicular bodies, compartment is crucial to identify their component receptors MVB). The distribution of the major CaMBP, shown by and ligands and follow the specific routes of each. Asialoglyco-Western blotting or by overlay with I 125 -calmodulin in proteins are almost completely cleared from circulation by the isolated fractions, was consistent with rapid accuthe hepatocytic asialoglycoprotein receptor. 7,8 After dissociamulation of I 125 -immunoglobulin A (IgA) in RRC and tion from the receptor within the endosomes, asialoglycopro-CURL after intravenous injection into rats. The receptor teins are degraded in lysosomes. 9-13 Asialoglycoproteins, such was also found in sinusoidal plasma membranes but not as asialofetuin (AF), therefore follow the classical endosomalin cell fractions containing apical (bile canalicular) or lysosomal pathway in hepatocytes. Polymeric immunoglobulateral plasma membrane domains of the hepatocyte.
lins, by contrast, undergo transcytosis from the sinusoidal to The interaction of pIgR with calmodulin was shown by the apical surface of the hepatocyte, where they are secreted direct binding assays and by affinity chromatography.
intact into bile. [14] The route followed by polymeric immu-Thus, calmodulin is the first cytoplasmic protein shown noglobulin A (pIgA) and its receptor (pIgR) from the basal to to interact with the pIgR. We postulate that calmodulin the apical surfaces is a paradigm for transcellular transregulates pIgA trafficking in rat liver. In addition, the port in hepatocytes and in Madin-Darby canine kidney receptor recycling fraction emerges as an endosomal (MDCK) cells. 19 However, the existence of specific transcytotic subcompartment involved in pIgA transport via pIgR.
vesicles or an apical recycling compartment is uncertain. 19 (HEPATOLOGY 1996;24:226-232.)
Recently, the pIgR has been shown to be a calmodulin-binding protein. This raises the possibility that Ca 2/ -calmodulin Hepatocytes internalize a wide variety of molecules from complexes play an important role in regulation of the transcythe circulation by receptor-mediated endocytosis. Once entotic route in the hepatocyte. docytosed, ligands and receptors are delivered to an ''early '' In this report we analyze the distribution of calmodulinendocytic compartment, the major site of sorting, where libinding proteins in rat liver endosomes and report two lines gands and receptors are forwarded towards their different of evidence that the receptor-recycling compartment (RRC) intracellular destinations. Lipoproteins, epidermal growth fraction may contain structures involved in transcytosis. factor (EGF), and asialoglycoproteins are taken up by hepato-
Methods
cytes and are finally degraded in lysosomes, whereas recycling receptors and some ligands, such as transferrin, are
Reagents. pIgA purified from rat myeloma cells (Lot No. recycled to the sinusoidal plasma membrane and are used 80601298) was from Zymed laboratories (San Francisco, CA). 17-a- for further rounds of internalization. 5 Ethinyl estradiol, calmodulin, and AF were from Sigma (St. Louis, MO).
Animals. Male Sprague-Dawley rats (250-300 g) maintained in standard chow were used in all the experiments. All animals received humane care in compliance with the institution's guidelines. In some Abbreviations: EGF, epidermal growth factor; LDL, low-density lipoprotein; AF, asial ofetuin; pIgA, polymeric immunoglobulin A; pIgR, polymeric immunoglobulin receptor; experiments animals were treated for 3 days with 17-a-ethinyl estra-MDCK, Madin-Darby canine kidney; RRC, receptor-recycling compartment; IgA, immuno-diol (1 mg/mL propylene glycol) as described to induce the expression globulin A; CURL, compartment of uncoupling of receptors and ligands; MVB, multivesicuof low density lipoprotein receptors. 21 lar bodies; SC, secretory component; PAGE, polyacrylamide gel electrophoresis; EGTA, Lipoproteins. Human LDL (1.025 õ d õ 1.050 g/mL) were isolated ethylene glycol-bis (b-aminoethyl ether) N,N,N,N-tetraacetic acid; SDS, sodium dodecyl from blood of normolipidemic human adults by sequential centrifugasulfate.