CHARACTERIZATION OF THE OSTEOCLAST VACUOLAR H+-ATPASE B-SUBUNIT

During bone resorption, osteoclasts acidify the extracellular bone resorbing compartment via a vacuolar H+-ATPase (V-ATPase), which resides in the ruffled-border membrane. In an effort to characterize the composition of the osteoclast V-ATPase catalytic domain, we have isolated a cDNA clone that encodes the V-ATPase B-subunit from a cDNA library constructed from highly purified chicken osteoclasts. Comparison of the predicted amino-acid sequence with the published sequences of isoforms of V-ATPase B-subunits from other sources revealed that the chicken osteoclast B-subunit is brain type and not kidney type. Furthermore, only clones encoding the brain type isoform of subunit B could be generated by PCR from a cDNA library prepared from human osteoclastoma osteoclast-like cells. Northern blot analysis revealed that two B-subunit mRNAs, approx. 1.7 and 3.5 kb in length, are expressed in chicken bone marrow mononuclear cells, brain and kidney, although the relative amounts of these two transcripts were different in each tissue. In brain, the 3.5-kb mRNA was predominantly expressed. In bone marrow cells, the levels of the 1.7-kb mRNA were higher than in other tissues and expression of this message was increased by 1,25-dihydroxyvitamin D-3, suggesting that this mRNA is specifically upregulated during osteoclast differentiation. These results indicate that the B-subunit isoforms present in the catalytic domains of the osteoclast and kidney V-H+-ATPases are different and further suggest that selective expression of isoforms of the B-subunit in these two tissues could provide a structural basis for some of the differences we have reported in the pharmacology and catalytic properties of these two enzymes.