Ram Rajasekharan

Department of Biochemistry
Research Areas: 
Lipid Metabolism, Signal Transduction in Plants, Bio-pesticides
Research Highlights: 

The long-term research goals of our lab are to understand lipid metabolism and signal transduction in plants. We are also interested in developing bio-pesticides. Our current projects include: (1) Molecular mechanism of Phosphoinositide-specific Phospholipase C (PLC) - Our work has demonstrated that the plant PLCs are soluble and that the C2 domain alone is capable of targeting plant PLC to the membrane in response to a Calcium signal. Furthermore, we have deciphered the mechanism by which plant PLC initiates signal transduction after Ca stimulation. We have also identified two conserved motifs in known plant PLC sequences, which are specific to plant PLC and therefore can be used to annotate plant PLC. (2) Regulation of lipid metabolism by PLC - We provide evidence for the regulation of lipid biosynthesis by phosphatidylinositol-specific phospholipase C (PLC) through UASino and two trans-acting elements. Using gene expression analysis and radiolabeling experiments, we have demonstrated that the overexpression of rice PLC in yeast cells can alter phospholipid biosynthesis at the levels of transcriptional and enzyme activity. This is the first report, implicating PLC in the direct regulation of lipid biosynthesis. These data suggest that regulation by PLC appears to be a mechanism for controlling the carbon flux toward phospholipid and sphingolipid synthesis by regulating either phosphatidic acid synthesis or the expression of genes involved in the utilization of phosphatidic acid. (3) Bifunctional oleosin is regulated by phosphorylation, a regulator of lipid metabolic functions - Triacylglycerol (TAG) is an important neutral lipid molecule that serves as the primary mechanism of fuel storage in eukaryotes. In plants, fatty oils are generally stored in spherical intracellular organelles referred to as oleosomes that are covered by proteins such as oleosin. We have isolated a catalytically active detergent resistance, 14 S multiprotein complex capable of acylating monoacylglycerol (MAG) from the microsomal membranes of developing peanut cotyledons and oleosin3 as a part of the complex. The recombinant OLE3 microsomes from Saccharomyces cerevisiae have been shown to have both a monoacylglycerol acyltransferase (MGAT) and a phospholipase A2 activity. A serine/threonine/tyrosine protein kinase phosphorylates oleosin. Using bimolecular fluorescence complementation analysis, we demonstrate that this kinase interacts with OLE3 and the fluorescence was associated with chloroplasts. Oleosin3-green fluorescent protein fusion protein exclusively associated with the chloroplasts. This is the first time our laboratory had shown that oleosin3 protein targeted to chloroplast. Phosphorylated OLE3 exhibited reduced monoacylglycerol acyltransferase and increased phospholipase A2 activities. Phosphorylation levels of OLE3 during seed germination were determined to be higher than in developing peanut seeds. Overexpression of the oleosin3 (OLE3) gene in S. cerevisiae resulted in an increased accumulation of diacylglycerol and triacylglycerols and decreased phospholipids. These findings provide a direct role for a structural protein (OLE3) in the biosynthesis and mobilization of plant oils.

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