A PLASMA-MEMBRANE SUCROSE-BINDING PROTEIN THAT MEDIATES SUCROSE UPTAKE SHARES STRUCTURAL AND SEQUENCE SIMILARITY WITH SEED STORAGE PROTEINSBUT REMAINS FUNCTIONALLY DISTINCT

Photoaffinity labeling of a soybean cotyledon membrane fraction identi fied a sucrose-binding protein (SEP). Subsequent studies have shown th at the SBP is a unique plasma membrane protein that mediates the linea r uptake of sucrose in the presence of up to 30 mM external sucrose wh en ectopically expressed in yeast, Analysis of the SEP-deduced amino a cid sequence indicates it lacks sequence similarity with other known t ransport proteins, Data presented here, however, indicate that the SEP shares significant sequence and structural homology with the vicilin- like seed storage proteins that organize into homotrimers. These simil arities include a repeated sequence that forms the basis of the reiter ated domain structure characteristic of the vicilin-like protein famil y, In addition, analytical ultracentrifugation and nonreducing SDS-pol yacrylamide gel electrophoresis demonstrate that the SEP appears to be organized into oligomeric complexes with a M-r indicative of the exis tence of SEP homotrimers and homodimers, The structural similarity sha red by the SEP and vicilin-like proteins provides st novel framework t o explore the mechanistic basis of SEP-mediated sucrose uptake, Expres sion of the maize Glb protein (a vicilin-like protein closely related to the SEP) in yeast demonstrates that a closely related vicilin-like protein is unable to mediate sucrose uptake, Thus, despite sequence an d structural similarities shared by the SEP and the vicilin-like prote in family, the SEP is functionally divergent from other members of thi s group.