Identification of a new murine eosinophil major basic protein (mMBP) gene: cloning and characterization of mMBP-2.

We have identified a new eosinophil major basic protein gene family member in the mouse and have given it the designation murine major basic protein-2 (mMBP-2). The gene was initially characterized as a unique expressed sequence tag (EST) clone having significant identity to the previously recognized member of this gene family, mMBP-1. The EST was used to screen and isolate mMBP-2 from a bone marrow cDNA library. In addition, a genomic clone of mMBP-2 was isolated and this gene was shown to be physically linked to within 100 kb of mMBP-1 on the central region of mouse chromosome 2. Progressive similarity alignment of the deduced mMBP-2 open reading frame demonstrates the apparent conservation of the "pre-pro-mature" protein structure found in the other known mammalian MBPs. Mature mMBP-2 maintains the cationic nature associated with these proteins with a predicted pI of 9.95. However, unlike the human MBPs, which display a three orders of magnitude charge difference [hMBP-1 (pI 11.4) vs. hMBP-2 (pI 8.7)], mMBP-2 is only slightly less cationic than mMBP-1 (pI 10.5). Expression studies demonstrate that transcription of the mMBP-2 gene parallels mMBP-1 and is confined to hematopoietic compartments engaged in eosinophilopoiesis. Moreover, using mMBP-1 knockout mice and immunohistochemistry with an antisera that recognizes both mMBP-1 and -2, we demonstrate that mMBP-2 protein expression is restricted to eosinophil lineage-committed cells.