Dimerization of the sodium/iodide symporter (NIS).

Author(s) Thompson, R.J.; Fletcher, A.; Brookes, K.; Nieto, H.; Alshahrani, M.M.; Mueller, J.W.; Fine, N.Hf; Hodson, D.J.; Boelaert, K.; Read, M.L.; Smith, V.Emma; McCabe, C.J.
Journal Thyroid
Date Published 2019 Jul 16
Abstract

Background The ability of thyroid follicular epithelial cells to accumulate iodide via the sodium/iodide symporter (NIS) is exploited to successfully treat most thyroid cancers, although a subset of patients lose functional NIS activity and become unresponsive to radioiodide therapy, with poor clinical outcome. Our knowledge of NIS regulation remains limited, however. Whilst numerous membrane proteins are functionally regulated via dimerization, there is little definitive evidence of NIS dimerization, and whether this might impact upon radioiodide uptake and treatment success is entirely unknown. We hypothesized that NIS dimerizes and that dimerization is a prerequisite for iodide uptake. Methods Co-immunoprecipitation, proximity ligation and Fӧrster resonance energy transfer (FRET) assays were used to assess NIS:NIS interaction. To identify residues involved in dimerization, a homology model of NIS structure was built based on the crystal structure of the dimeric bacterial protein vSGLT. Results Abundant cellular NIS dimerization was confirmed in vitro via three discrete methodologies. FRET and proximity ligation assays demonstrated that while NIS can exist as a dimer at the plasma membrane (PM), it is also apparent in other cellular compartments. Homology modelling revealed one key potential site of dimeric interaction, with 6 residues less than 3Å apart. In particular, NIS residues Y242, T243 and Q471 were identified as critical to dimerization. Individual mutation of residues Y242 and T243 rendered NIS non-functional, whilst abrogation of Q471 did not impact radioiodide uptake. FRET data show that the putative dimerization interface can tolerate the loss of one, but not two, of these three clustered residues. Conclusions We show for the first time that NIS dimerizes in vitro, and we identify the key residues via which this happens. We hypothesize that dimerization of NIS is critical to its trafficking to the PM, and may therefore represent a new mechanism which would need to be considered in overcoming therapeutic failure in patients with thyroid cancer.

DOI 10.1089/thy.2019.0034
ISSN 1557-9077
Citation Thompson RJ, Fletcher A, Brookes K, Nieto H, Alshahrani MM, Mueller JW, et al. Dimerization of the sodium/iodide symporter (NIS). Thyroid. 2019.

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