PROJECT SUMMARY Each day, respiratory surfaces are exposed to billions of particles whose accumulation could be harmful if not rapidly eliminated by mucociliary clearance. In health, mucus is thin and easily transported. In muco- obstructive diseases such as asthma, mucus is abundant, thick, and poorly transported. Mucus dysfunction is not adequately treated with existing therapies. To develop more effective therapies, it is essential to improve our mechanistic understanding of mucin biology. Two polymeric mucins MUC5AC/Muc5ac and MUC5B/Muc5b are the major macromolecules in airway mucus. They are very large glycoproteins that form even larger multi- unit polymers. During synthesis, polymeric mucins assemble via conserved carboxy and amino terminal domains also found in von Willebrand factor (VWF). Published reports on VWF have identified three C-terminal cysteines required for inter-chain dimerization in the endoplasmic reticulum (ER) and then transit into the Golgi. These cysteines are also present in MUC5AC/Muc5ac and MUC5B/Muc5b. After assembly in the ER, most proteins traffic in 90 nm diameter COPII coated vesicles that bud from the ER and get transported to the Golgi. However, individual mucin monomers are >500 nm long, so packaging assembled mucin dimers into vesicles for transport requires alternatives mechanisms. The objective of this proposal is to understand the polymerization and trafficking mechanisms of mucin from the ER to the Golgi. Aim 1 of this proposal (K99) will employ mice with point mutations in hypothesized cysteines to test their requirements in mucin assembly, ER to Golgi transport and secretion. In Aim 2 (K99/R00), the role of a non-conventional ER to Golgi trafficking component, MIA3, in trafficking mucin dimers will be determined using knock down approaches in human cell lines and primary cultures in health and disease. Aim 3 (R00) will determine the functions of MIA3 required for mucin ER to Golgi transport by interrogating its isoforms, specific domains, and binding partners, in human cell lines and primary cultures.. Overall, this work has the potential to identify novel mechanisms of mucin polymerization and mucin trafficking pathways in health and disease. This proposal takes advantage of the applicant’s expertise in mucin biology and vesicle trafficking. The additional training will help lead the candidate’s goal of establishing an independent lab studying mucin biology in health and disease.