N-glycosylation does not affect assembly and targeting of proglycinin in yeast

Glycinin, a simple protein, and beta-conglycinin, a glycoprotein, are the dominant storage proteins of soybean and are suggested to be derived from a common ancestor. To investigate why glycinin does not require glycosylation for its maturation, we attempted N-glycosylation of proglycinin A1aB1b using site-directed mutagenesis and yeast expression system. An N-glycosylation consensus sequence Asn-X-Ser/Thr was created at positions 103, 183, 196, 284 and 457 in the variable regions being strongly hydrophilic revealed from the alignment of amino acid sequences of various glycinin-type proteins. Among five mutant proglycinins (Q103N, H183N, G198T, S284N, N459T), Q103N was fully glycosylated, and H183N and N459T were partly (around 20% of the expressed proteins), whereas others were barely or not glycosylated. The glycosylated proglycinin was susceptible to endo-beta-N-acetylglucosamidase and N-glycanase cleavages. N-glycosylation did not cause inconveniences to processing of signal peptide, assembly into trimers and targeting into the vacuoles. Thermal and trypsin sensitivity analyses of the glycosylated proglycinin suggested that N-linked glycan prevents protein-protein interaction but does not stabilize the protein conformation. The reason why glycinin does not require N-glycosylation for its maturation is discussed.