Transport and processing of the glycosylated precursor of Concanavalin A in jack-bean.
Concanavalin A (ConA) is a tetrameric lectin which is synthesized in the developing cotyledons of jack bean (Canavalia ensiformis L.) as a glycosylated precursor, pro-concanavalin A (pro-ConA). The processing of pro-ConA involves the excision of a small glycopeptide from the center of the pro-ConA molecule, and the ligation of the two polypeptides. In this paper, we show that pro-ConA is associated with the endoplasmic reticulum/Golgi fraction of the cells, and that the processing of pro-ConA occurs in the protein bodies. Processing is a complex process and different intermediate-sized polypeptides appear at different times during cotyledon development. The ConA-related polypeptides which accumulate during seed development may be the products of alternate processing events or breakdown products of ConA, rather than precursors of ConA. When glycosylation is prevented by tunicamycin, there is very little transport of pro-ConA out of the endoplasmic reticulum/Golgi system to the protein bodies; the unglycosylated pro-ConA which is transported is slowly processed. Tunicamycin does not prevent the transport of canavalin (a protein which is not glycosylated) or the transport and processing of the small amounts of glycosylated pro-ConA synthesized in the presence of the drug. This is, to our knowledge, the first demonstration that the transport of a glycoprotein in plant cells is dependent on the presence of the glycan.
Department of Biology, University of California/San Diego, C-016, 92093, La Jolla, CA, USA.
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Chrispeels