Molecular cloning of POEM - A novel adhesion molecule that interacts with alpha(8)beta(1) integrin

Cell adhesion molecules are involved in a number of biological functions, s uch as cell survival, cell differentiation, tissue repair, and development. A novel molecule, POEM (preosteoblast epidermal growth factor-like repeat protein with meprin, A5 protein, and receptor protein-tyrosine phosphatase mu domain), was isolated by reverse transcription-polymerase chain reaction using a set of degenerate primers designed after other known epidermal gro wth factor (EGF)-like motifs. From its structure, POEM was suggested to be a novel adhesion molecule with five EGF-like domains, an Arg-GlyAsp (RGD) c ell binding motif, and a meprin, A5 protein, and receptor protein-tyrosine phosphatase mu (MAM) domain. By in situ hybridization using embryonic day 1 6.5 (E16.5) mouse embryos, strong expression of POEM mRNA was observed in d eveloping kidney renal tubules, parathyroid and thyroid glands, developing bone, tooth germ, and endocrine organs of the brain. The inner ear, skeleta l muscle, smooth muscle (except for the vascular system), and skin were als o positive for POEM expression. Bacterial recombinant POEM protein containi ng the RGD sequence and MAM domain showed strong cell adhesion, spreading, and survival-promoting activities. By mutating the RGD sequence to RGE, the cell spreading and survival activities were significantly decreased, but t he MAM domain was shown to contribute only to cell adhesion and not to cell . spreading and survival-promoting activities. The distribution of POEM in several tissues was close to that of alpha (8)beta (1) integrin. Therefore, we conducted cell adhesion assays using KA8 cells, a K562 leukemia clone s tably expressing alpha (8) integrin. Parental K562 cells, which expressed a lpha (5)beta (1) integrin, bound to fibronectin but not to POEM. On the oth er hand, KA8 cells showed strong binding and spreading on both fibronectin and POEM. These results suggest that POEM is a novel ligand for alpha (8)be ta (1) integrin and that POEM may be involved in the development and functi on of various tissues, such as kidney, bone, muscles, and endocrine organs.