A Novel Human Insulinoma-associated CDNA, IA-1, Encodes a Protein with "zinc-finger" DNA-binding Motifs

A subtraction library was constructed from human insulinoma (beta cell tumor) and glucagonoma (alpha cell tumor) cDNA phagemid libraries. Differential screening of 153 clones with end-labeled mRNAs from insulinoma, glucagonoma, and HeLa cells resulted in the isolation of a novel cDNA clone designated IA-1. This cDNA clone has a 2838-base pair sequence consisting of an open reading frame of 1530 nucleotides, which translates into a protein of 510 amino acids with a pI value of 9.1 and a molecular mass of 52,923 daltons. At the 3'-untranslated region there are seven ATTTA sequences between two polyadenylation signals (AATAAA). The IA-1 protein can be divided into two domains based upon the features of its amino acid sequence. The NH2-terminal domain of the deduced protein sequence (amino acids 1-250) has four classical pro-hormone dibasic conversion sites and an amidation signal sequence, Pro-Gly-Lys-Arg. The COOH-terminal domain (amino acids 251-510) contains five putative "zinc-finger" DNA-binding motifs of the form X3-Cys-X2-4-Cys-X12-His-X3-4-His-X4 which has been described as a consensus sequence for members of the Cys2-His2 DNA-binding protein class. Northern blot analysis revealed IA-1 mRNA in five of five human insulinoma and three of three murine insulinoma cell lines. Expression of this gene was undetectable in normal tissues. Additional tissue studies revealed that the message is expressed in several tumor cell lines of neuroendocrine origin including pheochromocytoma, medullary thyroid carcinoma, insulinoma, pituitary tumor, and small cell lung carcinoma. The restricted tissue distribution and unique sequence motifs suggest that this novel cDNA clone may encode a protein associated with the transformation of neuroendocrine cells.