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This Article Full Text Full Text (PDF) All Versions of this Article: 291/3/511    most recent 00274.2005v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Lizák, B. Articles by Bánhegyi, G. Search for Related Content PubMed PubMed Citation Articles by Lizák, B. Articles by Bánhegyi, G.

MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Translocon pores in the endoplasmic reticulum are permeable to small anions

¹Department of Medical Chemistry, Molecular Biology and Pathobiochemistry, Semmelweis University, ²Endoplasmic Reticulum Research Group of the Hungarian Academy of Sciences and Semmelweis University, Budapest, Hungary; and ³Dipartimento di Fisiopatologia, Medicina Sperimentale e Sanità Pubblica, Università di Siena, Siena, Italy

Submitted 10 June 2005 ; accepted in final form 31 March 2006

Contribution of translocon peptide channels to the permeation of low molecular mass anions was investigated in rat liver microsomes. Puromycin, which purges translocon pores of nascent polypeptides, creating additional empty pores, raised the microsomal uptake of radiolabeled UDP-glucuronic acid, while it did not increase the uptake of glucose-6-phosphate or glutathione. The role of translocon pores in the transport of small anions was also investigated by measuring the effect of puromycin on the activity of microsomal enzymes with intraluminal active sites. The mannose-6-phosphatase activity of glucose-6-phosphatase and the activity of UDP-glucuronosyltransferase were elevated upon addition of puromycin, but glucose-6-phosphatase and -glucuronidase activities were not changed. The increase in enzyme activities was due to a better access of the substrates to the luminal compartment rather than to activation of the enzymes. Antibody against Sec61 translocon component decreased the activity of UDP-glucuronosyltransferase and antagonized the effect of puromycin. Similarly, the addition of the puromycin antagonist anisomycin or treatments of microsomes, resulting in the release of attached ribosomes, prevented the puromycin-dependent increase in the activity. Mannose-6-phosphatase and UDP-glucuronosyltransferase activities of smooth microsomal vesicles showed higher basal latencies that were not affected by puromycin. In conclusion, translationally inactive, ribosome-bound translocons allow small anions to cross the endoplasmic reticulum membrane. This pathway can contribute to the nonspecific substrate supply of enzymes with intraluminal active centers.

puromycin; UDP-glucuronosyltransferase; glucose-6-phosphatase; -glucuronidase