BACKGROUND The mitochondrial pyruvate carrier (MPC) plays a central role in energy metabolism by transporting pyruvate across the inner mitochondrial membrane. Its heterodimeric composition and homology to SWEET and semiSWEET transporters set the MPC apart from the canonical mitochondrial carrier family (named MCF or SLC25). The import of the canonical carriers is mediated by the carrier translocase of the inner membrane (TIM22) pathway and is dependent on their structure, which features an even number of transmembrane segments and both termini in the intermembrane space. The import pathway of MPC proteins has not been elucidated. The odd number of transmembrane segments and positioning of the N-terminus in the matrix argues against an import via the TIM22 carrier pathway but favors an import via the flexible presequence pathway. RESULTS Here, we systematically analyzed the import pathways of Mpc2 and Mpc3 and report that, contrary to an expected import via the flexible presequence pathway, yeast MPC proteins with an odd number of transmembrane segments and matrix-exposed N-terminus are imported by the carrier pathway, using the receptor Tom70, small TIM chaperones, and the TIM22 complex. The TIM9·10 complex chaperones MPC proteins through the mitochondrial intermembrane space using conserved hydrophobic motifs that are also required for the interaction with canonical carrier proteins. CONCLUSIONS The carrier pathway can import paired and non-paired transmembrane helices and translocate N-termini to either side of the mitochondrial inner membrane, revealing an unexpected versatility of the mitochondrial import pathway for non-cleavable inner membrane proteins.

中文翻译：

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线粒体载体途径运输具有奇数个跨膜片段的非规范底物。

背景线粒体丙酮酸载体（MPC）通过跨线粒体内膜运输丙酮酸在能量代谢中发挥核心作用。其异二聚体组成以及与 SWEET 和半SWEET 转运蛋白的同源性使 MPC 有别于典型的线粒体载体家族（称为 MCF 或 SLC25）。经典载体的输入是由内膜载体转位酶（TIM22）途径介导的，并且取决于它们的结构，其特征是偶数个跨膜片段和膜间空间的两个末端。MPC蛋白的输入途径尚未阐明。奇数的跨膜片段和 N 末端在基质中的定位反对通过 TIM22 载体途径输入，但有利于通过灵活的前序列途径输入。结果在这里，我们系统地分析了 Mpc2 和 Mpc3 的输入途径，并报告说，与通过灵活的前序列途径的预期输入相反，具有奇数跨膜片段和基质暴露 N 末端的酵母 MPC 蛋白由载体输入途径，使用受体 Tom70、小 TIM 伴侣和 TIM22 复合物。TIM9·10 复合物使用保守的疏水基序通过线粒体膜间隙陪伴 MPC 蛋白，这些基序也是与经典载体蛋白相互作用所必需的。结论 载体途径可以导入配对和不配对的跨膜螺旋，并将 N 末端易位到线粒体内膜的任一侧，揭示了不可切割内膜蛋白的线粒体导入途径的意想不到的多功能性。