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Abstract: E3 ubiquitin ligases are central for the selection of proteins targeted for degradation by the ubiquitin proteasome pathway. In this study atrogin-1 (Fbox-32), a major E3 ligase in muscle, has been characterized in Atlantic salmon (Salmo salar). The protein sequence is highly conserved between teleosts and mammals and is characterized by the presence of five conserved motifs related to the identification of protein targets. The genomic structure is conserved between teleosts and mammals and contains 9 exon and 8 introns. The phylogenetic relationship between atrogin-1 and two other closely related ubiquitin E3 ligases FBXO25 and MuRF1 showed atrogin-1 and FBXO25 grouped together with MuRF1 being more distant. The mRNAs were expressed in multiple tissues, atrogin-1 and MuRF1 were most abundant in white muscle and heart whereas FBXO25 had greatest expression in brain, white muscle and heart. The transcriptional modulation of these E3 ligases was examined in starved fish and fish following different immune stimulations. Expression of atrogin-1 and MuRF1 was increased following food deprivation, implementing these two genes in degradation of muscle protein during starvation. During viral infection atrogin-1 expression was not altered, whereas it was increased following stimulation with LPS, indicating an onset of catabolic processes during inflammatory responses. [ABSTRACT FROM AUTHOR]