THE IDENTIFICATION AND CLONING OF A MURINE MAJOR BASIC-PROTEIN GENE EXPRESSED IN EOSINOPHILS

The existence of a murine homologue of the major basic protein (MBP) f ound in human eosinophil granules was initially hypothesized from stru ctural similarities at the electron microscopic level. The results pre sented in this study have extended these observations by describing th e identification/purification of a mouse MBP (mMBP) and the cloning of the gene encoding this eosinophil granule protein. Using protein puri fication methodologies with extravascular eosinophils, an mMBP homolog ue has been identified on the basis of strong (64%) N-terminal sequenc e homology with the mature human MBP (hMBP). Since hMBP results from a proteolytic cleavage of a precursor molecule, this sequence conservat ion suggests that the mouse granule protein is processed by a similar mechanism. The gene encoding mMBP was isolated using a hMBP cDNA clone as a heterologous probe in low criteria screens of mouse genomic and cDNA libraries. The genomic structure and nucleotide sequence of the m MBP exons are well conserved with the human gene, although homology al ignments of the encoded proteins show that extensive sequence conserva tion occurs only in the mature portion of the MBP molecules. Expressio n data demonstrate that this gene is transcriptionally active in tissu es containing eosinophil progenitor cells, such as femoral bone marrow . Genomic Southern blots using the mMBP gene at reduced stringency rev eal the potential existence of a second, more divergent MBP-like seque nce in the mouse. This suggests that, as with guinea pigs, the mouse g enome may also encode the eosinophil major basic protein from more tha n one gene.