The present invention relates to methods of facilitating the expression of recombinant polypeptides from cells, extracellular fluids, extracellular fibers, or any combination thereof, obtained from transgenic insect cells and larvae comprising a bacterial GlcNAc-6-P 2′-epimerase (GNPE), which is capable of converting N-acetyl-D-glucosamine-6-phosphate (GlcNAc-6-P) to N-acetyl-D-mannosamine-6-phosphate (ManNAc-6-P). The invention relates to methods to promote efficient glycoconjugate sialylation, by providing simpler ways to produce large intracellular pools of sialic acid precursors. The invention is also directed to nucleic acids, vectors, and cells comprising nucleic acids encoding polypeptides involved in the synthesis of sialic acid precursors, and cells in combination with nucleic acids encoding glycosyltransferases, including sialyltransferases, to facilitate the production of humanized recombinant glycoproteins. The engineered cells can be used to produce glycosylated proteins lepidopteran insects and cultured cell lines derived from Spodoptera frugiperda, Trichoplusia ni, and silkworms, such as Bombyx mori, particularly those that can be infected by baculovirus expression vectors.
