Identification of a second insulin-like growth factor in a fish species

An internal portion of insulin-like growth factor (IGF) amplified from the total cDNA of rainbow trout (Oncorhynchus mykiss) liver by a PCR was used to screen a rainbow trout liver cDNA library, and recombinant clones encoding two distinct IGFs were isolated. On the basis of a 98.7% nucleotide and 98.3% predicted amino acid identity to coho salmon IGF-I, one cDNA sequence was identified as rainbow trout preproinsulin-like growth factor I (rtIGF-I). The second cDNA sequence shared 46.1% and 43.3% identity with rtIGF-I at the nucleotide and predicted amino acid levels, respectively, and was identified as rainbow trout preproinsulin-like growth factor II (rtIGF-II). Predicted amino acid sequence comparisons of rtIGFs with those of human IGFs indicate that rtIGF-I is more similar to human IGF-I than to human IGF-II, and that rtIGF-II is more similar to human IGF-II than to human IGF-I. Southern blot analysis of rainbow trout genomic DNA probed with rtIGF-I and -II cDNA suggests that these two forms of IGF originate from separate genes. The presence of a teleost IGF-II suggests that the divergence of IGFs occurred early in vertebrate evolution.