Secretion of phytohemagglutinin by monkey COS cells

The entire coding region of a gene, which encodes a polypeptide of phytohemagglutinin (PHA-L), obtained from a library of genomic DNA of the common bean Phaseolus vulgaris cv. Greensleeves, was introduced into the SV40 expression vector pJC119. Monkey COS1 cells were transfected with the recombinant clone and the synthesis, glycosylation, and transport of PHA-L studied and compared with the normal processes in bean cotyledons. In the bean, phytohemagglutinin is synthesized on the rough endoplasmic reticulum and transported via the Golgi complex to protein bodies, vacuole-like organelles. Phytohemagglutinin was synthesized and glycosylated at the ER and processed in the Golgi apparatus of the transfected COS1 cells. After passing the Golgi apparatus, PHA-L was slowly secreted into the culture medium (half-time of 3-6 h), a result indicating that the signals for targeting proteins beyond the Golgi apparatus in plant cells are different from those in animal cells. PHA, which is stored in protein bodies in the plant cells, is secreted by animal cells. Tunicamycin inhibited both glycosylation and secretion of PHA by the COS1 cells, a finding indicating an essential role of the oligosaccharides for transport of PHA in these cells in contrast to the situation found in bean cotyledons. PHA, secreted into the culture medium, was partially sensitive to endo H, a result indicating the presence of one high-mannose and one complex oligosaccharide chain, a situation identical to that in beans.