Plasma membrane associated actin in liver of normal and tumour-bearing rats

A study was made of the association of actin with different plasma membrane fractions from liver of normal rats and from the enlarged liver of rats bearing a Walker 256 carcinoma where a decrease in the state of polymerisation of cytoplasmic actin has been previously observed. As estimated by the DNAase I inhibition assay, actin constituted approx. 7% and 3%, respectively, of the protein of membrane fractions enriched in lateral or bile-canalicular domains, but only trace amounts were found in the sinusoidal fraction. [3H]Cytochalasin B binding indicated the presence of 20 and 13 pmol of high-affinity binding sites per mg protein in lateral and bile-canalicular fractions, but none in the sinusoidal. Kd for cytochalasin B binding was of the order of 1 nM for lateral and bile-canalicular fractions. Polypeptide profiles obtained by SDS/urea/polyacrylamide gel electrophoresis of non-ionic detergent-insoluble residues differed for all three fractions although some proteins, including actin, occurred as major components of both bile-canalicular and lateral regions. Tumour growth had no effect on the actin content, high-affinity cytochalasin B binding or polypeptide profiles of the three membrane fractions.