Dias R O, Cardoso C, Pimentel A C, Damasceno T F, Ferreira C, Terra W R
Departamento de Bioquimica, Instituto de Quimica, Universidade de São Paulo, São Paulo, Brazil.
Insect Mol Biol. 2018 Feb;27(1):46-60. doi: 10.1111/imb.12340. Epub 2017 Aug 21.
Most insects have a gut lined with a peritrophic membrane (PM) consisting of chitin and proteins, mainly peritrophins that have chitin-binding domains. The PM is proposed to originate from mucus-forming mucins (Mf-mucins), which acquired a chitin-binding domain that interlocked with chitin, replacing mucus in function. We evaluated the expression of Mf-mucins and peritrophins by RNA-sequencing (RNA-seq) throughout the midgut of four distantly related insects. Mf-mucins were identified as proteins with high o-glycosylation and a series of uninterrupted Pro/Thr/Ser residues. The results demonstrate that the mucus layer is widespread in insects, and suggest that insect Mf-mucins are derived from those found in other animals by the loss of the cysteine knot and von Willebrand domains. The data also support a role of Mf-mucins in protecting the middle midgut of Musca domestica against acidic buffers. Mf-mucins may also produce a jelly-like material associated with the PM that immobilizes digestive enzymes in Spodoptera frugiperda. Peritrophins with a domain similar to Mf-mucins may be close to the ancestor of peritrophins. Expression data of peritrophins and chitin synthase genes throughout the midgut of M. domestica, S. frugiperda and Tenebrio molitor indicated that peritrophins were incorporated along the PM, according to their preferential sites of formation. Finally, the data support the view that mucus has functions distinct from the PM.
大多数昆虫的肠道内衬有一层围食膜（PM），它由几丁质和蛋白质组成，主要是具有几丁质结合结构域的围食蛋白。有人提出围食膜起源于形成黏液的黏蛋白（Mf - 黏蛋白），后者获得了一个与几丁质互锁的几丁质结合结构域，在功能上取代了黏液。我们通过RNA测序（RNA - seq）评估了四种亲缘关系较远的昆虫整个中肠中Mf - 黏蛋白和围食蛋白的表达。Mf - 黏蛋白被鉴定为具有高O - 糖基化和一系列不间断的脯氨酸/苏氨酸/丝氨酸残基的蛋白质。结果表明黏液层在昆虫中广泛存在，并表明昆虫的Mf - 黏蛋白是通过半胱氨酸结和血管性血友病因子结构域的缺失从其他动物中发现的黏蛋白衍生而来。数据还支持Mf - 黏蛋白在保护家蝇中肠中段免受酸性缓冲液影响方面的作用。Mf - 黏蛋白还可能产生一种与围食膜相关的果冻状物质，该物质可固定草地贪夜蛾中的消化酶。具有与Mf - 黏蛋白相似结构域的围食蛋白可能接近围食蛋白的祖先。家蝇、草地贪夜蛾和黄粉虫整个中肠中围食蛋白和几丁质合酶基因的表达数据表明，围食蛋白是根据其优先形成位点沿着围食膜整合的。最后，数据支持黏液具有与围食膜不同功能的观点。
