IDENTIFICATION OF BETA(2)-GLYCOPROTEIN-I AS A MEMBRANE-ASSOCIATED PROTEIN IN KIDNEY - PURIFICATION BY CALMODULIN AFFINITY-CHROMATOGRAPHY

Outer renal medulla calmodulin-binding proteins from a soluble protein fraction and a plasma membrane fraction solubilized in CHAPS were ret ained on a calmodulin-Sepharose 4B column in the presence of Ca2+, and subsequently eluted by EGTA. The calmodulin-binding proteins constitu ted 2.5% of the soluble protein and 0.1% of the solubilized membrane p rotein. beta(2)-glycoprotein I was identified as a calmodulin-binding protein both by N-terminal sequencing and by immunoblotting. Quantific ation showed that beta(2)-glycoprotein I constituted the major part (a pprox. 35%) of the calmodulin-binding membrane proteins, but only a mi nor part (approx. 0.1%) of the calmodulin-binding proteins in the solu ble fraction. These results show for the first time that beta(2)-glyco protein I binds calmodulin and that beta(2)-glycoprotein I may in kidn ey be a membrane-associated protein. Immunohistochemical studies ident ified beta(2)-glycoprotein I in several parts of the cortex and the me dulla of the kidney, including Bowman's capsula, the tubular lumen and the tubular epithelium, indicating that beta(2)-glycoprotein I, despi te its relatively high molecular mass, is filtrated in the glomerulus and subsequently reabsorbed by the tubular epithelium. This is in agre ement with beta(2)-glycoprotein I being a marker for renal tubular dis ease.