The invention provides methods and compositions for the rapid and sensitive detection of post-translationally modified proteins, and particularly of those with post-translational glycosylations. The methods can be used to detect O-GlcNAc posttranslational modifications on proteins on which such modifications were undetectable using other techniques. In one embodiment, the method exploits the ability of an engineered mutant of .beta.-1,4-galactosyltransferase to selectively transfer an unnatural ketone functionality onto O-GlcNAc glycosylated proteins. Once transferred, the ketone moiety serves as a versatile handle for the attachment of biotin, thereby enabling detection of the modified protein. The approach permits the rapid visualization of proteins that are at the limits of detection using traditional methods. Further, the preferred embodiments can be used for detection of certain disease states, such as cancer, Alzheimer's disease, neurodegeneration, cardiovascular disease, and diabetes.