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

In Arabidopsis pollen tubes, class I formins, transmembrane proteins, organize subarrays of actin during pollen tube elongation and show lateral mobility and localization dependent on O-glycosylation of their extracellular domain. In plant cells, linkage between the cytoskeleton, plasma membrane, and cell wall is crucial for maintaining 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 proline-rich extracellular domain and are candidate coordination factors. Here, using Arabidopsis, 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 extracellular domain of both is necessary for their function, and identify distinct O-glycans attached to these sequences, AtFH5 being hydroxyproline-arabinosylated and AtFH3 carrying arabinogalactan chains. Loss of hydroxyproline 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 in which AtFH5 and AtFH3, restricted to specific plasma membrane domains by their extracellular domains and the glycans attached to them, organize distinct subarrays of actin during pollen tube elongation.

Automated summary

In Arabidopsis pollen tubes, class I formins play a crucial role in organizing actin arrays during tube elongation, with their localization and mobility being dependent on O-glycosylation of their extracellular domain. The extracellular domain is necessary for the function of two pollen-expressed class I formins, AtFH3 and AtFH5, and specific O-glycans are attached to these sequences. Loss of O-glycosylation alters membrane localization and disrupts actin organization. The different O-glycans also affect lateral mobility in the plasma membrane. This study highlights the importance of protein sub-functionalization in pollen tube elongation.