Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794-5215, USA.
Department of Anesthesiology, Montefiore Medical Center, Albert Einstein College of Medicine, Bronx, NY 10467, USA.
Genetics. 2022 May 5;221(1). doi: 10.1093/genetics/iyac048.
Candida albicans cell wall glycoproteins, and in particular their mannose-rich glycans, are important for maintaining cellular integrity as well as host recognition, adhesion, and immunomodulation. The asparagine (N)-linked mannose outer chain of these glycoproteins is produced by Golgi mannosyltransferases (MTases). The outer chain is composed of a linear backbone of ∼50 α1,6-linked mannoses, which acts as a scaffold for addition of ∼150 or more mannoses in other linkages. Here, we describe the characterization of C. albicans OCH1, MNN9, VAN1, ANP1, MNN10, and MNN11, which encode the conserved Golgi MTases that sequentially catalyze the α1,6 mannose outer chain backbone. Candida albicans och1Δ/Δ, mnn9Δ/Δ, and van1Δ/Δ mutants block the earliest steps of backbone synthesis and like their Saccharomyces cerevisiae counterparts, have severe cell wall and growth phenotypes. Unexpectedly, and in stark contrast to S. cerevisiae, loss of Anp1, Mnn10, or Mnn11, which together synthesize most of the backbone, have no obvious deleterious phenotypes. These mutants were unaffected in cell morphology, growth, drug sensitivities, hyphal formation, and macrophage recognition. Analyses of secreted glycosylation reporters demonstrated that anp1Δ/Δ, mnn10Δ/Δ, and mnn11Δ/Δ strains accumulate glycoproteins with severely truncated N-glycan chains. This hypo-mannosylation did not elicit increased chitin deposition in the cell wall, which in other yeast and fungi is a key compensatory response to cell wall integrity breaches. Thus, C. albicans has evolved an alternate mechanism to adapt to cell wall weakness when N-linked mannan levels are reduced.
白色念珠菌细胞壁糖蛋白，尤其是其富含甘露糖的聚糖，对于维持细胞完整性以及宿主识别、黏附和免疫调节非常重要。这些糖蛋白的天冬酰胺（N）-连接甘露糖外链是由高尔基体甘露糖基转移酶（MTases）产生的。该外链由约 50 个α1,6 连接的甘露糖组成的线性骨架组成，作为添加其他连接方式约 150 个或更多甘露糖的支架。在这里，我们描述了 C. albicans OCH1、MNN9、VAN1、ANP1、MNN10 和 MNN11 的特征，这些基因编码顺序催化α1,6 甘露糖外链骨架的保守高尔基体 MTases。白色念珠菌 och1Δ/Δ、mnn9Δ/Δ 和 van1Δ/Δ 突变体阻断了骨架合成的最早步骤，与它们的酿酒酵母对应物一样，具有严重的细胞壁和生长表型。出乎意料的是，与酿酒酵母形成鲜明对比的是，失去合成大部分骨架的 Anp1、Mnn10 或 Mnn11 并没有明显的有害表型。这些突变体在细胞形态、生长、药物敏感性、菌丝形成和巨噬细胞识别方面没有受到影响。分泌糖基化报告基因的分析表明，anp1Δ/Δ、mnn10Δ/Δ 和 mnn11Δ/Δ 菌株积累了严重截短的 N-糖链的糖蛋白。这种低甘露糖化并没有引起细胞壁中几丁质沉积的增加，而在其他酵母和真菌中，这是细胞壁完整性破裂的关键补偿反应。因此，当 N-连接甘露聚糖水平降低时，白色念珠菌已经进化出一种替代机制来适应细胞壁的脆弱性。
