Molecular cloning and characterization of Kremen, a novel kringle-containing transmembrane protein

Kringle domain, a triple-disulfide-linked domain, is conserved in diverse proteins which play important roles in various biological processes. We cloned Kremen, a novel member of kringle-containing proteins, using a newly developed unique strategy, 'Kringle-SAGE (serial analysis of gene expression)', which enables comprehensive analysis of kringle-containing proteins. Kremen is likely to be a type-I transmembrane protein composed of 473 amino acid residues. Kremen has a kringle domain, a WSC domain, and CUB domains in the extracellular region, while the intracellular region has no conserved motif involved in signal transduction. In the mouse embryo, the Kremen mRNA level, which was increased during embryonic development, was localized in the apical ectodermal ridge of limb buds, myotome, and sensory organs (e.g. optic vesicle, otic vesicle, nasal pit). In the adult mouse, Kremen mRNA was expressed in a variety of tissues with a relatively strong expression in the lung, heart, and skeletal muscle. Kremen mRNA expression in C2C12 and NIE-115 cells increased during respective differentiation into muscular and neural cells. These results suggest a potential role for Kremen in the regulation of cellular responses upon extracellular stimulus or cell-cell interaction in neuronal and/or muscle cells. Kringle-SAGE is expected to facilitate further elucidation of structure and functions of kringle proteins.