Synthesis and processing of the dimorphic forms of rat α_2u-globulin

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 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.