Nucleolin is a calcium‐binding protein

We have purified a prominent 110‐kDa protein (p110) from 1.6 M NaCl extracts of rat liver nuclei that appears to bind Ca 2+ . p110 was originally identified by prominent blue staining with ‘Stains‐All’ in sodium dodecyl sulfate–polyacrylamide gels and was observed to specifically bind ruthenium red and 45 Ca 2+ in nitrocellulose blot overlays. In spin‐dialysis studies, purified p110 saturably bound approximately 75 nmol Ca 2+ /mg protein at a concentration of 1 mM total Ca 2+ with half‐maximal binding observed at 105 μM Ca 2+ . With purification, p110 became increasingly susceptible to proteolytic (likely autolytic) fragmentation, although most intermediary peptides between 40 and 90 kDa retained “Stains‐All”, ruthenium red, and 45 Ca 2+ binding. N‐terminal sequencing of intact p110 and a 70‐kDa autolytic peptide fragment revealed a strong homology to nucleolin. Two‐dimensional sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE)/IEF revealed autolysis produced increasingly acidic peptide fragments ranging in apparent pI's from 5.5 for intact p110 to 3.5 for a 40 kDa peptide fragment. Intact p110 and several peptide fragments were immunostained with a highly specific anti‐nucleolin antibody, R2D2, thus confirming the identity of this protein with nucleolin. These annexin‐like Ca 2+ ‐binding characteristics of nucleolin are likely contributed by its highly acidic argyrophilic N‐terminus with autolysis apparently resulting in largely selective removal of its basic C‐terminal domain. Although the Ca 2+ ‐dependent functions of nucleolin are unknown, we discuss the possibility that like the structurally analogous HMG‐1, its Ca 2+ ‐dependent actions may regulate chromatin structure, possibly during apoptosis. J. Cell. Biochem. 85: 268–278, 2002. © 2002 Wiley‐Liss, Inc.