Duplicated zebrafish insulin‐like growth factor binding protein‐5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity

Insulin‐like growth factor binding protein (IGFBP)‐5 is a secreted protein that binds to IGF and modulates IGF actions. IGFBP‐5 is also found in the nucleus of mammalian cells and has transactivation activity. The structural basis of this transactivation activity and its role in mediating IGF‐independent actions are not clear. Here we report that there are 2 igfbp‐5 genes in zebrafish and other teleost fish. In zebrafish, igfbp‐5a and ‐5b are expressed in spatially restricted, mostly nonoverlapping domains during early development. The IGF binding site is conserved in both zebrafish IGFBP‐5s, and they are both secreted and capable of IGF binding. Both proteins contain a consensus bipartite nuclear localization signal and were found in the nucleus when introduced into cultured cells. Although zebrafish IGFBP‐5b possesses transactivation activity, zebrafish IGFBP‐5a lacks this activity. Mutational analysis demonstrated that 2 unique amino acids in positions 22 and 56 of IGFBP‐5a are responsible for its lack of transactivation activity. These findings suggest that the duplicated zebrafish IGFBP‐5s have evolved divergent regulatory mechanisms and distinct biological properties by partitioning of ancestral structural domains and provide new evidence for a conserved role of the IGF binding, nuclear localization, and transactivation domain of this multifunctional IGFBP.—Dai, W., Kamei, H., Zhao, Y., Ding, J., Du, Z., Duan, C. Duplicated zebrafish insulin‐like growth factor binding protein‐5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity. FASEB J. 24, 2020–2029 (2010). www.fasebj.org