Most eukaryotic transmembrane and secreted proteins contain N-terminal signal peptides that mediate insertion of the nascent translation products into the membrane of the endoplasmic reticulum. After membrane insertion, signal peptides typically are cleaved from the mature protein and degraded. Here, we tested whether a small hydrophobic protein selected for growth promoting activity in mammalian cells retained transforming activity while also acting as a signal peptide. We replaced the signal peptide of the PDGF β receptor (PDGFβR) with a previously described 29-residue artificial transmembrane protein named 9C3 that can activate the PDGFβR in trans. We showed that a modified version of 9C3 at the N-terminus of the PDGFβR can function as a signal peptide, as assessed by its ability to support high level expression, glycosylation, and cell surface localization of the PDGFβR. The 9C3 signal peptide retains its ability to interact with the transmembrane domain of the PDGFβR and cause receptor activation and cell proliferation. Cleavage of the 9C3 signal peptide from the mature receptor is not required for these activities. However, signal peptide cleavage does occur in some molecules, and the cleaved signal peptide can persist in cells and activate a co-expressed PDGFβR in trans. Our finding that a hydrophobic sequence can display signal peptide and transforming activity suggest that some naturally occurring signal peptides may also display additional biological activities by interacting with the transmembrane domains of target proteins.

大多数真核跨膜蛋白和分泌蛋白含有 N 末端信号肽，这些信号肽介导新生翻译产物插入内质网膜。膜插入后，信号肽通常从成熟蛋白上切割下来并降解。在这里，我们测试了选择用于哺乳动物细胞中的生长促进活性的小型疏水蛋白是否保留了转化活性，同时还充当信号肽。我们用先前描述的名为 9C3 的 29 残基人工跨膜蛋白替代了 PDGF β 受体 (PDGFβR) 的信号肽，该蛋白可以反式激活PDGFβR. 我们发现 PDGFβR N 末端的 9C3 修饰版本可以作为信号肽发挥作用，通过其支持 PDGFβR 的高水平表达、糖基化和细胞表面定位的能力来评估。9C3 信号肽保留了与 PDGFβR 的跨膜结构域相互作用并引起受体激活和细胞增殖的能力。这些活动不需要从成熟受体切割 9C3 信号肽。然而，信号肽切割确实发生在某些分子中，切割的信号肽可以在细胞中持续存在并激活反式共表达的 PDGFβR. 我们发现疏水序列可以显示信号肽和转化活性，这表明一些天然存在的信号肽也可能通过与靶蛋白的跨膜结构域相互作用而显示出额外的生物活性。