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Christian de Caestecker, a Ph.D. student in the lab of Ian Macara, Louise B. McGavock Professor and chair of the Department of Cell and Developmental Biology, has proposed and validated a mechanism that addresses a decades-old mystery surrounding epithelial cells. de Caestecker's research, published in Nature Cell Biology, sheds light on the process by which epithelial cells, polarized cells that face the outside world, sort and deliver the specialized proteins they need at each cell's top (outermost) surface.

Epithelial cells are organized like boxes, with tops and sides called the apical and lateral surfaces. To properly perform their functions, epithelial cells must accurately sort proteins to each surface, and defects in the delivery of such proteins can play a role in multiple human diseases such as cancers.

"Most human cancers are epithelial in origin and show defects in polarized membrane protein distributions," Macara said. "How delivery occurs to the top, or apical, surface has remained a mystery for decades, but Christian's brilliance and hard work led to an important breakthrough."

Freshly synthesized proteins are sent to the Golgi for additional processing and for sorting to their final destinations through a process analogous to the postal service. Delivery to the sides of the cell is well understood, owing to the presence of "zip codes" in proteins that instruct the cell which path the protein should take to the cell surface.

Apical proteins lack equivalent zip codes, so the apical sorting process has remained more elusive. Considering that many apical membrane proteins have very short or non-existent cytoplasmic domains—meaning that most of their bulk resides within or outside of the plasma membrane and not inside the cell—de Caestecker hypothesized that proteins destined to go to the apical membrane are sorted at the Golgi by the physical size of the cytoplasmic domains.

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