Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. Sucrose synthase (SuSy) is a glycosyl transferase enzyme that plays a key role in sugar metabolism, primarily in sink tissues. SuSy catalyzes the reversible cleavage of sucrose into fructose and either uridine diphosphate glucose (UDP-G) or adenosine diphosphate glucose (ADP-G). The products of sucrose cleavage by SuSy are available for many metabolic pathways, such as energy production, primary-metabolite production, and the synthesis of complex carbohydrates. SuSy proteins are usually homotetramers with an average monomeric molecular weight of about 90 kD (about 800 amino acids long). Plant SuSy isozymes are mainly located in the cytosol or adjacent to plasma membrane, but some SuSy proteins are found in the cell wall, vacuoles, and mitochondria. Plant SUS gene families are usually small, containing between four to seven genes, with distinct exon-intron structures. Plant SUS genes are divided into three separate clades, which are present in both monocots and dicots. A comprehensive phylogenetic analysis indicates that a first SUS duplication event may have occurred before the divergence of the gymnosperms and angiosperms and a second duplication event probably occurred in a common angiosperm ancestor, leading to the existence of all three clades in both monocots and dicots. Plants with reduced SuSy activity have been shown to have reduced growth, reduced starch, cellulose or callose synthesis, reduced tolerance to anaerobic-stress conditions and altered shoot apical meristem function and leaf morphology. Plants overexpressing SUS have shown increased growth, increased xylem area and xylem cell-wall width, and increased cellulose and starch contents, making SUS high-potential candidate genes for the improvement of agricultural traits in crop plants. This review summarizes the current knowledge regarding plant SuSy, including newly discovered possible developmental roles for SuSy in meristem functioning that involve sugar and hormonal signaling.

Skin fibroblasts from a patient w gous familial hypercholesterolemia (HFH) were with normal skin fibroblasts with regard to bindii ization, and degradation of iodinated human low poprotein (LDL). Like other cell lines from HF the mutant cells showed no suppression of sterol s LDL. Surface binding, measured at O0 to elimir preciable internalization that was shown to occur on the average slightly less for HFH cells than X at low LDL concentrations but comparable or e at high LDL concentrations (>60 ,gg of LDL prot A major defect observed was in the rate of intern LDL at 370, which was only 1-10% of that in n LDL degradation was also markedly reduced bu same extent. Thus, a larger fraction of the LDL t peared to be degraded by the mutant cells. The n defect observed, then, was not in surface binding in rate of LDL internalization. While this migi dary to a defect in specific binding sites of LDL tude of the observed differences in binding at Il ture seems too small to account for the huge dil internalization (13-to 115-fold).

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Aortic smooth muscle cells from the rat were successfully grown in tissue culture and shown to have characteristic morphology. '"I-labeled homologous very low density lipoproteins and high density lipoproteins were taken up by these smooth muscle cells during incubation for 48 hours at the stationary phase. Despite multiple washings, a large proportion of the lipoprotein radioactivity associated with the cells was apparently surface bound and trypsin releasable. With both lipoprotein fractions, lipid and protein uptake by the cells measured after trypsinization was related to time and to the amount of lipoprotein protein added to the medium. Compared with protein, there was a disproportionately greater entry of lipid radioactivity into the cells. Light and electron microscope autoradiography localized the label intracellularly over the cell cytoplasm, cell boundaries, and, in some cells, over lysosomes. On the basis of either protein uptake or whole particle uptake, approximately four times as much high density lipoprotein as very low density lipoprotein was taken up by the smooth muscle cells. To assess metabolism and degradation of high density lipoproteins, aortic smooth muscle cells were incubated in fresh unlabeled medium for 48 hours after exposure to '"I-labeled high density lipoproteins. A large proportion of radioactivity released was trichloroacetic acid precipitable, suggesting some release of whole lipoprotein protein; however, these lipoproteins appeared to be modified when they were tested with anti-high density lipoprotein antiserum. Also, water-soluble radioactivity (presumably protein breakdown products) was released in amounts that averaged 3% of the protein label in the cells. These results indicate that although aortic smooth muscle cells growing in tissue culture can rapidly take up lipids and lipoproteins, catabolism of lipoprotein protein is slow. Correlative biochemical and ultrastructural analysis suggests the possibility of regurgitation of noncatabolized lipoprotein protein by reverse endocytosis. KEY WORDShigh density lipoproteins electron microscopy very low density lipoproteins immunoprecipitation aging atherosclerosis autoradiography trypsinization• In previous studies the synthetic (1-5) and the degradative (6-9) pathways of lipid metabolism have been investigated using both whole arteries and cell-free homogenates. These experiments have provided evidence that aortas of various species can utilize different precursors for the