The low-density lipoprotein receptors perform diverse cellular roles ranging from endocytosis and cargo transport to subcellular trafficking and cell signaling. Despite this diversity in function, lipoprotein-related receptors (LRPs) possess an extracellular domain comprised of common repeating clusters of cysteine-rich, complement-like repeats (ligand binding domains), EGF repeats, and YWTD containing domains that form beta-propeller structures. The number and arrangement of these motifs varies amongst receptors. The complex structure and high number of disulfide bonds in LRP family members pose a formidable posttranslational processing challenge to cells. The correct trafficking of LRPs and disposal of improperly folded receptors is assisted by many proteins and enzymes in the secretory pathway. While the majority of these proteins participate in general quality control, cell culture studies provide evidence that the novel MESD protein targets the LRP6 propeller. If MESD is essential for folding and trafficking the propeller domain of related LRPs, mutations in mesd could have wide ranging effects not only on development but also for adult physiology. Consistent with this prediction, embryos lacking mesd fail to form a primitive streak and differentiate mesoderm. Combined, embryology, genetics and biochemistry provide a powerful approach towards understanding the function of this novel protein. In order to begin to address mesd function, we will: (I) Determine how the mesd phenotype relates to LRP signaling by comparison of the mesd deficient and LRP5-/-; LRP6-/- phenotypes and utilize chimera analysis to identify tissues requiring mesd function; (II) Determine if MESD facilitates folding and localization of other LRPs in cell culture; (III) Determine if MESD is essential for LRP folding in vivo by examining genetic dosage interaction with LRP mutations and tissue specific knockout of mesd; and finally, (IV) Identify regions of MESD important for function in ES cell differentiation and transgenic mice. Combined, embryology, genetics and biochemistry provide a powerful approach towards understanding the function of this novel protein.