MasABK Proteins Interact with Proteins of the Type IV Pilin System to Affect Social Motility of Myxococcus xanthus

Gliding motility is critical for normal development of spore-filled fruiting bodies in the soil bacterium Myxococcus xanthus. Mutations in mgl block motility and development but one mgl allele can be suppressed by a mutation in masK, the last gene in an operon adjacent to the mgl operon. Deletion of the entire 5.5 kb masABK operon crippled gliding and fruiting body development and decreased sporulation. Expression of pilAGHI, which encodes type IV pili (TFP) components essential for social (S) gliding, several cryptic pil genes, and a LuxR family protein were reduced significantly in the Δmas mutant while expression of the myxalamide operon was increased significantly. Localization and two-hybrid analysis suggest that the three Mas proteins form a membrane complex. MasA-PhoA fusions confirmed that MasA is an integral cytoplasmic membrane protein with a ≈100 amino acid periplasmic domain. Results from yeast two-hybrid assays showed that MasA interacts with the lipoprotein MasB and MasK, a protein kinase and that MasB and MasK interact with one another. Additionally, yeast two-hybrid analysis revealed a physical interaction between two gene products of the mas operon, MasA and MasB, and PilA. Deletion of mas may be accompanied by compensatory mutations since complementation of the Δmas social gliding and developmental defects required addition of both pilA and masABK.

滑动运动（gliding motility）对于土壤细菌黄色粘球菌（Myxococcus xanthus）产孢子实体的正常发育至关重要。mgl基因的突变会阻断滑动运动与发育进程，但其中一种mgl等位基因的缺陷可被masK基因的突变所抑制，而masK是紧邻mgl操纵子的另一操纵子中的最后一个基因。完整删除全长5.5 kb的masABK操纵子会损害滑动运动与子实体发育能力，并降低孢子形成效率。在Δmas突变株中，编码参与社会（S型）滑动运动必需的IV型菌毛（type IV pili, TFP）组分的pilAGHI基因、多个功能未知的pil基因以及一个LuxR家族蛋白的表达水平显著下调，而myxalamide操纵子的表达则显著上调。定位实验与双杂交分析表明，三种Mas蛋白可形成膜结合复合物。MasA与PhoA的融合蛋白实验证实，MasA是一类整合型细胞质膜蛋白，拥有约100个氨基酸残基的周质结构域。酵母双杂交（yeast two-hybrid）实验结果显示，MasA可与脂蛋白MasB以及蛋白激酶MasK发生相互作用，且MasB与MasK之间也存在相互作用。此外，酵母双杂交分析还发现，mas操纵子编码的两种基因产物MasA、MasB与PilA之间存在物理相互作用。由于要互补修复Δmas突变株的社会滑动运动与发育缺陷，需要同时补充pilA与masABK基因，因此mas基因的缺失可能伴随有补偿性突变。