Signal transduction pathway is initiated by the interaction between signaling receptors on the target cell and signal molecules secreted by the producing cell. Although fundamentally important, it remains largely unclear how newly synthesized signal molecules are secreted and how plasma membrane targeted transmembrane signal receptors are delivered to the plasma membrane to perform their physiological functions. The endoplasmic reticulum (ER) is an important sorting station in the secretory transport pathway. Secretory proteins get folding and modifications in the ER, export from the ER and go through the subsequent trafficking routes towards their destinations. Here, we elucidated the ER export process of a transmembrane signal receptor, Frizzled-6 (Fzd6) and a soluble secretory signal...[ Read more ]

Signal transduction pathway is initiated by the interaction between signaling receptors on the target cell and signal molecules secreted by the producing cell. Although fundamentally important, it remains largely unclear how newly synthesized signal molecules are secreted and how plasma membrane targeted transmembrane signal receptors are delivered to the plasma membrane to perform their physiological functions. The endoplasmic reticulum (ER) is an important sorting station in the secretory transport pathway. Secretory proteins get folding and modifications in the ER, export from the ER and go through the subsequent trafficking routes towards their destinations. Here, we elucidated the ER export process of a transmembrane signal receptor, Frizzled-6 (Fzd6) and a soluble secretory signaling protein, sonic hedgehog (Shh). Fzd6 regulates planar cell polarity (PCP), a process during which cells are polarized along the plane of the epithelium. PCP plays important roles in developmental process and organ function. We found that the delivery of Fzd6 to the cell surface is regulated by two conserved polybasic motifs, which directly interact with the secretion-associated Ras-related GTPase 1A (SAR1A) subunit of coat protein complex II (COPII). This interaction is critical for enrichment of Fzd6 into COPII vesicles to be exported out of the ER. Moreover, we found that newly synthesized Fzd6 is associated with another PCP protein, cadherin EGF LAG seven-pass G-type receptor 1 (CELSR1), and that this association regulates their surface delivery, providing a novel mechanism to ensure the appropriate stoichiometry of these PCP proteins on cell boundaries. The signaling pathway initiated by Shh plays important roles in various developmental processes in metazoan. We found that cargo receptor Surfeit locus protein 4 (SURF4) regulates the ER export of Shh by directly interacting with the Cardin- Weintraub (CW) motif in Shh. The CW motif is known to interact with proteoglycans (PGs) that are predominantly synthesized at the Golgi. Interestingly, we found that PGs compete with SURF4 to bind N fragment of Shh (ShhN), and that inhibiting synthesis of PGs causes defects in the export of ShhN from the trans Golgi network (TGN). SURF4 and PG maturation are also important for surface delivery of full length Shh in mammalian cells. In summary, our study reveals a SURF4-to-PG relay mechanism that regulates the sorting and secretion of Shh. Our results provide clear mechanistic insights into the trafficking machinery that regulates the ER export of Fzd6 and Shh in the secretory transport pathway. The novel protein interactions that are identified in our study to be critical for the ER export of Shh or Fzd6 will provide new drug targets to downregulate the signaling transduction pathway regulated by these cellular factors.