Identities ascribed to the intact protein ions detected in MALDI-MS of whole bacterial cells or from other complex mixtures are often ambiguous. Isolation of candidate proteins can establish that they are of correct molecular mass and sufficiently abundant, but by itself is not definitive. An in vivo labeling strategy replacing methionine with selenomethionine has been employed to deliver an additional constraint for protein identification, i.e., number of methionine residues, derived from the shift in mass of labeled versus unlabeled proteins. By stressing a culture and simultaneously labeling, it was possible to specifically image the cells' response to the perturbation. Because labeled protein is only synthesized after application of the stress, it provides a means to view dynamic changes in the cellular proteome. These methods have been applied to identify a 15,879 Da protein ion from E. coli that was induced by an antibacterial agent with an unknown mechanism of action as SpY, a stress protein produced abundantly in spheroplasts. It has also allowed us to propose protein identities (and eliminate others from consideration) for many of the ions observed in MALDI (and ESI-MS) whole cell profiling at a specified growth condition.

中文翻译：

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体内标记：动态蛋白质组的一瞥和蛋白质鉴定的其他限制。

在整个细菌细胞或其他复杂混合物的MALDI-MS中检测到的完整蛋白质离子的身份通常不明确。候选蛋白的分离可以确定它们具有正确的分子质量并且足够丰富，但是其本身不是确定的。已经采用体内标记策略用硒代蛋氨酸代替蛋氨酸来提供蛋白质鉴定的附加限制，即，蛋氨酸残基的数量，其是由标记的蛋白质相对于未标记的蛋白质的质量变化得出的。通过强调培养并同时进行标记，可以对细胞对微扰的反应进行专门成像。因为标记的蛋白质仅在施加压力后才合成，所以它提供了一种查看细胞蛋白质组动态变化的方法。这些方法已被用于从大肠杆菌中鉴定出15879 Da的蛋白质离子，该离子由具有未知作用机理的抗菌剂诱导为SpY，SpY是原生质球中大量产生的应激蛋白。它也使我们能够为特定生长条件下在MALDI（和ESI-MS）全细胞谱图中观察到的许多离子提出蛋白质同一性（并从考虑中消除其他因素）。