THE PLANT CELL, Vol 4, Issue 12 1561-1574, Copyright © 1992 by American Society of Plant Biologists
A 62-kD Sucrose Binding Protein Is Expressed and Localized in Tissues Actively Engaged in Sucrose Transport
H. D. Grimes, P. J. Overvoorde, K. Ripp, V. R. Franceschi and W. D. Hitz
Department of Botany, Washington State University, Pullman, Washington 99164-4238
Sucrose transport from the apoplasm, across the plasma membrane, and into
the symplast is critical for growth and development in most plant species.
Phloem loading, the process of transporting sucrose against a concentration
gradient into the phloem, is an essential first step in long-distance
transport of sucrose and carbon partitioning. We report here that a soybean
62-kD sucrose binding protein is associated with the plasma membrane of
several cell types engaged in sucrose transport, including the mesophyll
cells of young sink leaves, the companion cells of mature phloem, and the
cells of the developing cotyledons. Furthermore, the temporal expression of
the gene and the accumulation pattern of the protein closely parallel the
rate of sucrose uptake in the cotyledon. Molecular cloning and sequence
analysis of a full-length cDNA for this 62-kD sucrose binding protein
indicated that the protein is not an invertase, contains a 29-amino acid
leader peptide that is absent from the mature protein, and is not an
integral membrane protein. We conclude that the 62-kD sucrose binding
protein is involved in sucrose transport, but is not performing this
function independently.
