The cytoplasmic domain of the beta subunit of the insulin-like growth facto
r I receptor (amino acids 936-1337) was overexpressed in Sf9 insect cells u
sing a baculovirus expression system, and the g-His tagged receptor was pur
ified by metal-affinity chromatography. Autophosphorylation of the receptor
was concentration dependent, consistent with a trans phosphorylation mecha
nism, Phosphoamino acid analysis of the autophosphorylated receptor showed
predominantly phosphotyrosine, but phosphoserine and phosphothreonine were
also present. However, when the receptor was further purified by gel filtra
tion on Sephadex G-100 and then autophosphorylated, phosphoaminoacid analys
is showed only phosphotyrosine, We conclude that the IGF-I receptor tyrosin
e kinase is not a dual-specificity kinase and that autophosphorylation of t
he beta subunit is by a trans mechanism.