Chinese hamster ovary (CHO) cells express only a subset of the glycosyltransferase activities known to exist. They do not express several fucosyltransferases, galactosyltransferases, sialyltransferases or N-acetylglucosaminyltransferases. However, following mutagenesis or transfection with large amounts of DNA, rare mutants that express a transferase activity de novo have been obtained. The first CHO mutant of this type was LEC10, which expresses the N-acetylglucosaminyltransferase, GlcNAc-TIII, that adds the bisecting GIcNAc to complex N-glycans. Several analogous gain-of-function mutants have now been characterized and, all express a new glycosyltransferase activity. In several cases, expression is known to reflect gene activation at the transcriptional level. Thus, CHO cells contain quiescent glycosyltransferase genes that may be activated by mutational events. Several of these transferases have properties distinct from previously described enzymes. In fact, the most recently characterized dominant CHO mutants, LEC14 and LEC18, each express a GlcNAc-T activity that creates novel N-glycans never before observed in glycoproteins from any other source. In these and possibly other cases, it appears the CHO genome has provided access to new GlcNAc-Ts that may be difficult to identify by conventional methods.
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