O-glycosylation of the extracellular domain of pollen class I formins modulates their plasma membrane mobility

In plant cells, linkage between the cytoskeleton, plasma membrane and cell wall is crucial to maintain cell shape. In highly polarized pollen tubes, this coordination is especially important to allow rapid tip-growth and successful fertilization. Class I formins contain cytoplasmic actin-nucleating formin homology domains as well as a Pro-rich extracellular domain (ECD) and are candidate coordination factors. Here, we investigated the functional significance of the extracellular domain of two pollen-expressed class I formins: AtFH3, which does not have a polar localization and AtFH5, which is limited to the growing tip region. We show that the ECD of both is necessary for their function and identify distinct O-glycans attached to these sequences, AtFH5 being Hyp-arabinosylated and AtFH3 carrying arabinogalactan chains. Loss of Hyp-arabinosylation altered the plasma membrane localization of AtFH5 and disrupted actin cytoskeleton organization. Moreover, we show that O-glycans differentially affect lateral mobility in the plasma membrane. Together, our results support a model of protein sub-functionalization where AtFH5 and AtFH3, restricted to specific plasma membrane domains by their ECDs and the glycans attached to them, organize distinct subarrays of actin during pollen tube elongation.