Insights Into the Mechanism of MCT8 Oligomerization

Mutations in the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in MCT8 deficiency, characterized by severe intellectual and motor disability. The MCT8 protein is predicted to have 12 transmembrane domains (TMDs) and is expressed as monomers, homodimers, and homo-oligomers. This study aimed to delineate the mechanism of MCT8 oligomerization. Coimmunoprecipitation studies demonstrated that lithium dodecyl sulfate effectively disrupts MCT8 protein complexes, indicating the involvement of non-covalent interactions. Successive C-terminal truncations of the MCT8 protein altered the oligomerization pattern only if introduced in the N-terminal half of the protein (TMD1-6). The truncation at extracellular loop 1 (E206X) still allowed homodimerization, but completely abrogated homo-oligomerization, whereas both were preserved by the C231X mutant (at TMD2), suggesting that the minimally required oligomerization sites are located proximal of Cys231. However, mutant constructs lacking the intracellular N-terminus or TMD1 and 2 were still capable to form homo-oligomers. Therefore, other domains distal of Cys231 are also likely to be involved in the formation of extensive multidomain interactions. This hypothesis was supported by structural modeling. Despite multiple approaches, MCT8 oligomerization could not be fully abrogated unless a substantial part of the protein was removed, precluding detailed studies into its functional role. Together, our findings suggest that MCT8 oligomerization involves extensive noncovalent interactions between the N-terminal halves of MCT8 proteins. Most mutations identified in patients with MCT8 deficiency have only minor effects on MCT8 oligomerization and, thus, impaired oligomerization does not appear to be an important pathogenic mechanism.

Bioproduction of Citric Acid Exposed to Lithium Dodecyl Sulfate

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Binding and acid-base properties of novel photosensitizing drugs in micellar and liposome solutions

The binding and acid-base equilibria of the two novel mesoporphyrin derivatives, PB07 and PB109 (quino[4,4a,5,6-efg]- annulated 7-demethyl-8-deethylmesoporphyrin and 2'-cyano-8'-formyl-N'-methyl-1',1a',5a',6'-tetrahydroacrido [4,5,5a,6-bcd]- annulated 2,3-dihydromesoporphyrin, resp.), which are promising agents for photodynamic therapy (PDT), were studied in aqueous solutions of different surfactants (Triton X-100 (TX-100), dodecyl maltoside (DDM), cetyltrimethylammonium bromide (CTAB), lithium dodecyl sulfate (LDS)) and phosphatidyl choline (PC) liposomes. In all cases, the porphyrins are solubilized at neutral/alkaline pH in monomeric form and remain micelle/liposome-bound independently of their ionization state. The dissociation constants of the solubilized porphyrins are found to be influenced by the charge of the surface groups of the carrier. The protonation of pyrrole/quinoline nitrogens of the studied porphyrins is facilitated in the following order: LDS ≫TX-100 (DDM, PC liposomes) > CTAB. The dissociation constants of PB 07 carboxylic groups are similar in neutral/cationic micellar and liposome solutions and are significantly decreased for LDS-bound pigment. The results provide necessary information for the optimization of delivery systems for PB 07 and PB 109 when applied as sensitizers in PDT.