Lunatic fringe protein processing by proprotein convertases may contribute to the short protein half-life in the segmentation clock

During vertebrate segmentation, oscillatory activation of Notch signaling is important in the clock that regulates the timing of somitogenesis. In mice, the cyclic activation of NOTCH1 requires the periodic expression of Lunatic fringe (Lfng). For LFNG to play a role in the segmentation clock, its cyclic transcription must be coupled with post-translational mechanisms that confer a short protein half-life. LFNG protein is cleaved and released into the extracellular space, and here we examine the hypothesis that this secretion contributes to a short LFNG intracellular half-life, facilitating rapid oscillations within the segmentation clock. We localize N-terminal protein sequences that control the secretory behavior of fringe proteins and find that LFNG processing is promoted by specific proprotein convertases including furin and SPC6. Mutations that alter LFNG processing increase its intracellular half-life without preventing its secretion. These mutations do not affect the specificity of LFNG function in the Notch pathway, thus regulation of protein half-life affects the duration of LFNG activity without altering its function. Finally, the embryonic expression pattern of Spc6 suggests a role in terminating LFNG activity during somite patterning. These results have important implications for the mechanisms that contribute to the tight control of Notch signaling during vertebrate segmentation.