A novel and highly divergent homolog of human eosinophil granule major basic protein

Eosinophils are important effector cells in defense against helminth infect ion and in allergic diseases. To identify novel eosinophil proteins, large scale sequencing of a cDNA library prepared from interleukin-5-stimulated u mbilical cord precursor cells was performed, and the major genes expressed by maturing eosinophils were determined. This resulted in the identificatio n of a cDNA with 64% identity to human prepro-major basic protein (hprepro- MBP). This cDNA was designated hprepro-MBP homolog (hprepro-MBPH). Interest ingly, the calculated pI values for hMBPH and hMBP differed by > 100-fold, with pI values of 8.7 and 11.4, respectively, Given this pronounced basicit y difference, the homolog transcript's abundance (1.1%), and MBP's critical role in eosinophil biological activity, we further characterized the homol og. Reverse transcription-polymerase chain reaction detected transcription of hprepro-MBPH in bone marrow only, and this result was confirmed by analy sis of a large cDNA data base (electronic Northern). hMBPH was isolated fro m human eosinophil granule lysates, and its identity was verified by amino acid sequencing and by mass spectrometry. Analyses of the biological activi ties showed that hMBPH had effects similar to hMBP in cell killing and neut rophil (superoxide anion production and interleukin-8 release) and basophil (histamine and leukotriene C-4 release) stimulation assays, but usually wi th reduced potency. Overall, this novel homolog's unique physical propertie s indicated that the high net positive charge of hMBP is important but not essential for biological activity.