Altered RNA Binding Activity Underlies Abnormal Thyroid Hormone Metabolism Linked to a Mutation in Selenocysteine Insertion Sequence-binding Protein 2

Jodi L Bubenik,Donna M Driscoll

doi:10.1074/jbc.m707059200

Abstract

The expression of selenoproteins requires the translational recoding of the UGA stop codon to selenocysteine. In eukaryotes, this requires an RNA stem loop structure in the 3'-untranslated region, termed a selenocysteine insertion sequence (SECIS), and SECIS-binding protein 2 (SBP2). This study implicates SBP2 in dictating the hierarchy of selenoprotein expression, because it is the first to show that SBP2 distinguishes between SECIS elements in vitro. Using RNA electrophoretic mobility shift assays, we demonstrate that a naturally occurring mutation in SBP2, which correlates with abnormal thyroid hormone function in humans, lies within a novel, bipartite RNA-binding domain. This mutation alters the RNA binding affinity of SBP2 such that it no longer stably interacts with a subset of SECIS elements. Assays performed under competitive conditions to mimic intracellular conditions suggest that the differential affinity of SBP2 for various SECIS elements will determine the expression pattern of the selenoproteome. We hypothesize that the selective loss of a subset of selenoproteins, including some involved in thyroid hormone homeostasis, is responsible for the abnormal thyroid hormone metabolism previously observed in the affected individuals.

Highlights

Selenium is an essential micronutrient for human health
Short term interaction of SECIS-binding protein 2 (SBP2) with the selenocysteine insertion sequence (SECIS) would serve to prime the system for selenocysteine insertion, and the complex would be transferred to L30 on the ribosome for multiple rounds of translation
Because SBP2 is a limiting factor in selenocysteine insertion, this model would allow for greater selenoprotein synthesis than one that requires constant association between SBP2 and the SECIS

Summary

Introduction

Selenium is an essential micronutrient for human health. Most of the selenium in the body is found as selenocysteine, which is incorporated into proteins termed selenoproteins. Equimolar amounts of wild type and mutant proteins were tested using an in vitro translation assay employing a luciferase reporter whose expression is dependent on selenocysteine insertion.

Results

Conclusion