Matthias Schleiden Institute / Genetics, Friedrich Schiller University Jena, Philosophenweg 12, 07743 Jena, Germany.
Leibniz Institute on Aging - Fritz Lipmann Institute, Beutenbergstr. 11, 07745 Jena, Germany.
Mol Plant Microbe Interact. 2020 Sep;33(9):1129-1141. doi: 10.1094/MPMI-02-20-0028-R. Epub 2020 Jul 14.
Phytoplasmas are intracellular bacterial plant pathogens that cause devastating diseases in crops and ornamental plants by the secretion of effector proteins. One of these effector proteins, termed SECRETED ASTER YELLOWS WITCHES' BROOM PROTEIN 54 (SAP54), leads to the degradation of a specific subset of floral homeotic proteins of the MIKC-type MADS-domain family via the ubiquitin-proteasome pathway. In consequence, the developing flowers show the homeotic transformation of floral organs into vegetative leaf-like structures. The molecular mechanism of SAP54 action involves binding to the keratin-like domain of MIKC-type proteins and to some RAD23 proteins, which translocate ubiquitylated substrates to the proteasome. The structural requirements and specificity of SAP54 function are poorly understood, however. Here, we report, based on biophysical and molecular biological analyses, that SAP54 folds into an α-helical structure. Insertion of helix-breaking mutations disrupts correct folding of SAP54 and compromises SAP54 binding to its target proteins and, concomitantly, its ability to evoke disease phenotypes in vivo. Interestingly, dynamic light scattering data together with electrophoretic mobility shift assays suggest that SAP54 preferentially binds to multimeric complexes of MIKC-type proteins rather than to dimers or monomers of these proteins. Together with data from literature, this finding suggests that MIKC-type proteins and SAP54 constitute multimeric α-helical coiled coils. Our investigations clarify the structure-function relationship of an important phytoplasma effector protein and may thus ultimately help to develop treatments against some devastating plant diseases.
植原体是一种能在农作物和观赏植物中引起严重疾病的细胞内细菌病原体，它通过分泌效应蛋白来实现这一目标。这些效应蛋白中的一种，被称为“分泌的星状黄花丛枝蛋白 54（SAP54）”，通过泛素-蛋白酶体途径导致 MIKC 型 MADS 结构域家族中特定亚组花同源蛋白的降解。结果，发育中的花朵表现出花器官向类似叶片的营养叶结构的同源转化。SAP54 作用的分子机制涉及与 MIKC 型蛋白的角蛋白样结构域以及某些 RAD23 蛋白的结合，这些蛋白将泛素化的底物转运到蛋白酶体中。然而，SAP54 功能的结构要求和特异性还了解甚少。在这里，我们根据生物物理和分子生物学分析报告，SAP54 折叠成一个α-螺旋结构。插入螺旋破坏突变会破坏 SAP54 的正确折叠，并损害 SAP54 与其靶蛋白的结合能力，同时也损害其在体内引发疾病表型的能力。有趣的是，动态光散射数据和电泳迁移率变动分析表明，SAP54 优先结合 MIKC 型蛋白的多聚体复合物，而不是这些蛋白的二聚体或单体。结合文献中的数据，这一发现表明 MIKC 型蛋白和 SAP54 构成多聚体α-螺旋卷曲螺旋。我们的研究阐明了一种重要的植原体效应蛋白的结构-功能关系，因此最终可能有助于开发针对一些严重植物疾病的治疗方法。
