A plasma membrane sucrose-binding protein that mediates sucrose uptake shares structural and sequence similarity with seed storage proteins but remains functionally distinct

Photoaffinity labeling of a soybean cotyledon membrane fraction identified a sucrose-binding protein (SEP). Subsequent studies have shown that the SBP is a unique plasma membrane protein that mediates the linear uptake of sucrose in the presence of up to 30 mM external sucrose when ectopically expressed in yeast, Analysis of the SEP-deduced amino acid sequence indicates it lacks sequence similarity with other known transport 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 similarities include a repeated sequence that forms the basis of the reiterated domain structure characteristic of the vicilin-like protein family, In addition, analytical ultracentrifugation and nonreducing SDS-polyacrylamide gel electrophoresis demonstrate that the SEP appears to be organized into oligomeric complexes with a M-r indicative of the existence of SEP homotrimers and homodimers, The structural similarity shared by the SEP and vicilin-like proteins provides st novel framework to explore the mechanistic basis of SEP-mediated sucrose uptake, Expression 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 and structural similarities shared by the SEP and the vicilin-like protein family, the SEP is functionally divergent from other members of this group.