Linkages between protein ubiquitination, proteasome activity and the expression of oxygen-binding proteins in Antarctic notothenioid fishes

Abstract

Antarctic icefishes lack hemoglobin (Hb), and some species lack cardiac myoglobin (Mb). As iron-centered proteins, Hb and Mb can promote the formation of reactive oxygen species that may damage biological macromolecules. Consistent with this, we find higher levels of oxidized proteins in some tissues of red-blooded notothenioids than in icefishes. Oxidized proteins are marked for degradation by the conjugation of the protein ubiquitin. I hypothesized that levels of ubiquitinated proteins and 20S proteasome activity (which degrades oxidized proteins) would be higher in +Hb and +Mb notothenioids than icefishes lacking the proteins. Levels of ubiquitinated proteins and rates of proteasome activity were measured in the heart ventricle, pectoral adductor, and liver of six species of notothenioids differing in Hb and Mb expression. Previous studies in notothenioids suggest that oxidative stress declines following acclimation to 4°C. I also hypothesized that levels of ubiquitinated proteins and 20S proteasome activity would decline in response to acclimation to 4°C. Levels of ubiquitinated proteins and rates of proteasome activity were measured in the heart ventricle, pectoral adductor, and liver of the red-blooded Notothenia coriiceps held at ambient temperature and acclimated to 4°C for 3 weeks. Levels of ubiquitinated proteins were higher in tissues of the red-blooded N. coriiceps compared to icefishes, but the activity of the 20S proteasome did not follow a similar trend, suggesting that icefishes do not incur an energetic benefit resulting from reduced rates of protein degradation. Levels of ubiquitinated proteins were equivalent in heart ventricle and oxidative skeletal muscle, and proteasome activities were equivalent in all tissues between acclimated N. coriiceps and those held at ambient temperature, suggesting that protein damage and rates of protein degradation are not altered in notothenioids by long-term exposure to 4°C.