Biosynthesis, post-translation modification, and functional characterization of Drm/Gremlin

Down-regulated by mos (Drm)/Gremlin is a highly conserved protein whose pro perties and expression pattern suggest a role in early development, tissue- specific differentiation, and cell transformation. We have investigated the biosynthesis and processing of Drm expressed endogenously in rat fibroblas ts or overexpressed following transient or stable transfection. Analysis of metabolically labeled cells revealed that Drm exists in secreted and cell- associated forms that exhibit similar mobilities in SDS-polyacrylamide gel electrophoresis. Protein analysis indicated that Drm is present in two majo r species: a slow migrating glycosylated form and a nonglycosylated form. B oth forms of Drm are able to undergo phosphorylation. Drm is released into the media within 30 min of synthesis and is detectable for up to 4-5 h, whe reas the cell-associated form has a half-life of about 1 h, Confocal immuno fluorescent microscopy indicates that Drm is present both on the external s urface of expressing cells, as well as within the endoplasmic reticulum and the Golgi. Both glycosylated and nonglycosylated forms of Drm exhibit iden tical distributions and are able to antagonize bone morphogenetic protein s ignaling. Like the soluble form, the cell-associated forms are capable of b inding I-125-bone morphogenetic protein-4. These properties are consistent with a role for Drm in interfering with signaling and indicate that Drm may act at the cell surface during tissue development and transformation.