Synthesis and processing of the dimorphic forms of rat α 2u-globulin

Abstract

α 2u-Globulin, the androgen-dependent male rat urinary protein, can be resolved into two distinct molecular forms by SDS-polyacrylamide slab gel electrophoresis. These two forms designated as α 2uA ( M r 18 800) and α 2uB ( M r 18 100) are found both in urine and in the liver cells. Translation of rat liver mRNA in the rabbit reticulocyte lysate produced two preprotein forms of α 2u-globulin, designated as α 2uA′ ( M r 20 300) an α 2uB′ ( M r 19 600). Cell-free translation of rat liver mRNA in the presence of dog pancreas microsomal membrane or in Xenopus oocytes produced the two processed forms of α 2u-globulin (α 2uA and α 2uB). Quantitation of α 2uA and α 2uB within the in vitro translation products of the hepatic mRNA from albino rats of Yale, Sprague-Dawley and Fischer strains showed genetic differences in the proportion of translatable mRNA for α 2uA and α 2uB. The ratio of α 2uA: α 2uB in the translation products of liver mRNA from Yale rats was found to be 1:2.5 while in the case of both Sprague-Dawley and Fischer rats, the ratio was 1:4. A small portion of the α 2uA and α 2uB synthesized in the cultured hepatocytes, in the Xenopus oocytes or in the membrane-supplemented cell-free system appeared as two additional forms, designated as α 2uA′ ( M r 21 200) and α 2uB′ ( M r 20 600). Unlike α 2uA and α 2uB both α 2uA′ and α 2uB′ were found to bind to Con A-Sepharose, suggesting their glycoprotein nature.