An agouti mutation lacking the basic domain induces yellow pigmentation but not obesity in transgenic mice

Chronic antagonism of melanocortin receptors by the paracrine-acting agouti gene product induces both yellow fur and a maturity-onset obesity syndrome in mice that ubiquitously express wild-type agouti. Functional analysis of agouti mutations in transgenic mice indicate that the cysteine-rich C terminus, signal peptide, and glycosylation site are required for agouti activity in vivo. In contrast, no biological activity has been ascribed to the conserved basic domain. To examine the functional significance of the agouti basic domain, the entire 29-aa region was deleted from the agouti cDNA, and the resulting mutation (agoutiΔbasic) was expressed in transgenic mice under the control of the β-actin promoter (BAPaΔbasic). Three independent lines of BAPaΔbasic transgenic mice all developed some degree of yellow pigment in the fur, indicating that the agoutiΔbasic protein was functional in vivo. However, none of the BAPaΔbasic transgenic mice developed completely yellow fur, obesity, hyperinsulinemia, or hyperglycemia. High levels of agoutiΔbasic expression in relevant tissues exceeded the level of agouti expression in obese viable yellow mice, suggesting that suboptimal activity or synthesis of the agoutiΔbasic protein, rather than insufficient RNA synthesis, accounts for the phenotype of the BAPaΔbasic transgenic mice. These findings implicate a functional role for the agouti basic domain in vivo, possibly influencing the biogenesis of secreted agouti protein or modulating protein–protein interactions that contribute to effective antagonism of melanocortin receptors.