Identification of INSL6, a new member of the insulin family that is expressed in the testis of the human and rat

A new member of the insulin gene family (INSL6) was identified from an Expr essed Sequence Tag database through a search for proteins containing the in sulin family B-chain cysteine motif. Human and rat INSL6 encoded polypeptid es of 213 and 188 amino acids, respectively. These orthologous sequences co ntained the B-chain, C-peptide, and A-chain motif found in other members of the insulin family Human lNSL6 was 43% identical to human relaxin H2 in th e B- and A-chain regions. As with other family members, human and rat INSL6 had predicted dibasic sequences at the junction of the C-peptide and A-cha in. Human INSL6 sequence had an additional dibasic site near the C-terminus of the A-chain. The presence of a single basic residue at the predicted ju nction of the B-chain and C-peptide suggests that multiple prohormone conve rtases are required to produce the fully mature hormone. INSL6 was found to be expressed at high levels in the testis as determined by Northern blot a nalysis and specifically within the seminiferous tubules in spermatocytes a nd round spermatids as detected by in situ hybridization analysis. Radiatio n hybrid mapping placed the human INSL6 locus at chromosome 9p24 near the p lacenta insulin-like homologue INSL4 and the autosomal testis-determining f actor (TDFA) locus.