Overexpressed LIM mineralization proteins do not require LIM domains to induce bone

Rat LIM mineralization protein 1 (LMP-1, an LIM domain protein) mediates bone morphogenetic protein 6 (BMP-6) induction of bone nodule formation in fetal rat calvarial osteoblast (ROB) cultures. We have isolated the complementary DNA (cDNA) for the human homologue of LMP-1 from an adult human heart cDNA library and showed that when overexpressed it is osteoinductive in the same culture system. The recently, revised cDNA sequence of Enigma, the protein product of which binds to the insulin receptor and the tyrosine kinase receptor ret, now matches the nucleotide sequence of human LMP-1 (hLMP-1). A truncated, 223 amino acid (AA) LMP-1(t) protein has identical effects as the full-length protein, despite the deletion of the LIM domains. Two splice variants of human LMP-1 have been detected. Human LMP-2 has a 119-base pair (bp) deletion between bp 325 and 444 and a 17-bp insertion at bp 444. The resulting derived protein contains 423 AA with the LIM domains intact and does not induce bone formation when overexpressed in ROB cultures. Human LMP-3 has the same 17 nucleotide insertion at bp 444, resulting in a shift in the reading frame that causes a stop codon to occur at bp 505-507. The resulting 153 AA protein does not have the LIM domains, but overexpression of hLMP-3 induces bone formation in osteoblast cultures. These findings suggest that the LIM domains are not required for LMPs to induce bone formation. In addition, a small region (36 AA) of the LMP-1 protein may be required for bone formation.