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Papers In Press, published online ahead of print May 16, 2005
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Internal Medicine/Gastroenterology, Virginia Commonwealth University, Richmond, VA 23298-0341
Corresponding Author: William.Pandak{at}med.va.gov
Recently identified StarD5 belongs to the StarD4 subfamily; a subfamily of steroidogenic acute regulatory related lipid transfer (START) domain proteins that includes StarD4 and StarD6; proteins whose functions remain unknown. The objective of this study was to confirm StarD5’s protein localization and sterol binding capabilities as measures to pursue function. Using rabbit polyclonal antibody against newly purified human his-tagged/StarD5 protein, StarD5 protein was detected in human liver. In parallel studies, increased expression of StarD5 in primary hepatocytes led to a marked increase in microsomal free-cholesterol. Cell fractionation studies demonstrated StarD5 protein in liver cytosolic fractions only, suggesting StarD5 as a directional cytosolic sterol carrier. Supportive in vitro binding assays demonstrated a concentration dependent binding of cholesterol by StarD5 similar to that of the cholesterol binding START domain protein, StarD1. In contrast to selective cholesterol binding by StarD1, StarD5 bound the potent regulatory oxysterol, 25-hydroxycholesterol, in a concentration dependent fashion. StarD5 binding appeared selective for cholesterol and 25-hydroxycholesterol as no binding was observed for other tested sterols. The ability of StarD5 to bind not only cholesterol, but also 25-hydroxycholesterol, a potent inflammatory mediator and regulatory oxysterol, raises basic fundamental questions about StarD5’s role in the maintenance of cellular cholesterol homeostasis.
Revised on May 12, 2005
Accepted on May 12, 2005
Human StarD5, a cytosolic StAR-related lipid binding protein
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