MILK-COMPOSITION AND LACTATION OF BETA-CASEIN-DEFICIENT MICE

beta-Casein is a major protein component of milk and, in conjunction w ith the other caseins, it is assembled into micelles. The casein micel les determine many of the physical characteristics of milk, which are important for stability during storage and for milk-processing propert ies. There is evidence that suggests that beta-casein may also possess other, nonnutritional functions. To address the function of beta-case in, the mouse beta-casein gene was disrupted by gene targeting in embr yonic stem cells. Homozygous beta-casein mutant mice are viable and fe rtile; females can lactate and successfully rear young. beta-Casein wa s expressed at a reduced level in heterozygotes and was completely abs ent from the milk of homozygous mutant mite. Despite the deficiency of beta-casein, casein micelles were assembled in heterozygous and homoz ygous mutants, albeit with reduced diameters. The absence of beta-case in expression was reflected in a reduced total protein concentration i n milk, although this was partially compensated for by an increased co ncentration of other proteins. The growth of pups feeding on the milk of homozygous mutants was reduced relative to those feeding on the mil k of wild-type mice. Various genetic manipulations of caseins have bee n proposed for the qualitative improvement of cow's milk composition. The results presented here demonstrate that beta-casein has no essenti al function and that the casein micelle is remarkably tolerant of chan ges in composition.