Insect proteins homologous to mammalian high mobility group proteins I/Y (HMG I/Y). Characterization and binding to linear and four-way junction DNA.

A chromosomal high mobility group (HMG) protein from an epithelial cell line of Chironomus tentans (Diptera) was purified to homogeneity and chemically characterized. cDNA clones encoding this protein were isolated and sequenced. The deduced amino acid sequence revealed a high similarity to HMG protein I of mammalia. This insect protein has therefore been designated cHMGI. It has a deduced molecular mass of 10,371 kDa and appears to be a product of a single gene copy. Similarly to mammalian HMGI/Y proteins the insect cHMGI protein has three putative DNA-binding motifs with a sequence K/RXRGRP that are each encoded by one exon of the gene. Using synthetic peptides we have shown that the first and the second motif are necessary for high-affinity binding of the protein to DNA. Protein cHMGI binds preferentially to AT-rich DNA with a half-saturation value of 1.1 nM. Both cHMGI and human HMGI proteins recognize specifically a four-way junction DNA. We have also purified a related protein with similar physico-chemical properties from Drosophila melanogaster Kc cells. The identification and characterization of HMGI proteins in insects with polytene chromosomes and with the cytologic and genetic potential of Chironomus and Drosophila opens new possibilities for studying function(s) of this group of chromosomal proteins.