Surface delivery quantification reveals distinct trafficking efficiencies among clustered protocadherin isoforms.

Proteins that transmit molecules and signals across the plasma membrane are crucial in cell biology because they enable cells to sense and respond to their surroundings. A major challenge for studying cell surface proteins is that often they do not fold or traffic properly to the plasma membrane when produced in heterologous cells. We developed a strategy for quantifying surface localization from fluorescence microscopy images of surface-stained cells. Using clustered protocadherins, a protein family important for cell-cell recognition during neuronal development, we found that surface delivery levels vary among clustered protocadherin isoforms and between wild-type and engineered variants. Quantifying these differences provides evidence that cis dimerization is not tightly coupled to surface delivery for clustered protocadherins. This work establishes a generalizable framework for screening proteins and variants of interest for proper cell surface localization.