MOLECULES AT THE EXTERNAL NUCLEAR SURFACE

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Abstract
The molecules occurring as terminal residues on the external surfaces of nuclei prepared from rat liver by either sucrose-CaCl2 or citric acid methods and nucleoli derived from the sucrose-CaCl2 nuclei were studied chemically and electrokinetically. In 0.0145 M NaCl, 4.5% sorbitol, and 0.6 mM NaHCO3 with pH 7.2 ± 0.1 at 25°C, the sucrose-CaCl2 nuclei had an electrophoretic mobility of -1.92 µm/s/V/cm, the citric acid nuclei, -1.63 µm/s/V/cm, and the nucleoli, -2.53 µm/s/V/cm. The citric acid nuclei and the nucleoli contained no measurable sialic acid. The sucrose-CaCl2 nuclei contained 0.7 nmol of sialic acid/mg nuclear protein; this was essentially located in the nuclear envelope. Treatment of these nuclei with 50 µg neuraminidase/mg protein resulted in release of 0.63 nmol of sialic acid/mg nuclear protein; treatment with 1 % trypsin caused release of 0.39 nmol of the sialic acid/mg nuclear protein. The pH-mobility curves for the particles indicated the sucrose-CaCl2 nuclei surface had an acid-dissociable group of pK. ∼2.7 while the pK for the nucleoli was considerably lower. Nucleoli treated with 50 µg neuraminidase/mg particle protein had a mobility of -2.53 µm/s/V/cm while sucrose-CaCl2 nuclei similarly treated had a mobility of -1.41 µm/s/V/cm. Hyaluronidase at 50 µg/mg protein had no effect on nucleoli mobility but decreased the sucrose-CaCl2 nuclei mobility to -1.79 µm/s/V/cm. Trypsin at 1 % elevated the electrophoretic mobility of the sucrose-CaCl2 nuclei slightly but decreased the mobility of the nucleoli to -2.09 µm/s/V/cm. DNase at 50 µg/mg protein had no effect on the mobility of the isolated sucrose-CaCl2 nuclei but decreased the electrophoretic mobility of the nucleoli to -1.21 µm/s/V/cm. RNase at 50 µg/mg protein also had no effect on the electrophoretic mobility of the sucrose-CaCl2 nuclei but decreased the nucleoli mobility to -2.10 µm/s/V/cm. Concanavalin A at 50 µg/mg protein did not alter the nucleoli electrophoretic mobility but decreased the sucrose-CaCl2 nuclei electrophoretic mobility to -1.64 µm/s/V/cm. The results are interpreted to mean that the sucrose-CaCl2 nuclear external surface contains terminal sialic acid residues in trypsin-sensitive glycoproteins, contains small amounts of hyaluronic acid, is completely devoid of nucleic acids, and binds concanavalin A. The nucleolus surface is interpreted to contain a complex made up of protein, RNA, and primarily DNA, to be devoid of sialic acid and hyaluronic acid, and not to bind concanavalin A.