Catalyzes the transfer of the GlcNAc-1-phosphate moiety from UDP-GlcNAc onto the carrier lipid undecaprenyl phosphate (C55-P), yielding GlcNAc-pyrophosphoryl-undecaprenyl (GlcNAc-PP-C55). di-trans,octa-cis-undecaprenyl phosphate + UDP-N-acetyl-alpha-D-glucosamine = N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UMP Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. A transposon mutant in wecA/rfe allows HeLa cell invasion but no cell spreading. Shows decreased secretion of IscA, a protein required for cell spreading. The lipopolysaccharide produced is of a size consistent with it containing the core component but no O-antigen side chain. Belongs to the glycosyltransferase 4 family. WecA subfamily.