A pollen protein, NaPCCP, that binds pistil arabinogalactan proteins also binds phosphatidylinositol 3-phosphate and associates with the pollen tube endomembrane system.

As pollen tubes grow toward the ovary, they are in constant contact with the pistil extracellular matrix (ECM). ECM components are taken up during growth, and some pistil molecules exert their effect inside the pollen tube. For instance, the Nicotiana alata 120-kD glycoprotein (120K) is an abundant arabinogalactan protein that is taken up from the ECM; it has been detected in association with pollen tube vacuoles, but the transport pathway between these compartments is unknown. We recently identified a pollen C2 domain-containing protein (NaPCCP) that binds to the carboxyl-terminal domain of 120K. As C2 domain proteins mediate protein-lipid interactions, NaPCCP could function in intracellular transport of 120K in pollen tubes. Here, we describe binding studies showing that the NaPCCP C2 domain is functional and that binding is specific for phosphatidylinositol 3-phosphate. Subcellular fractionation, immunolocalization, and live imaging results show that NaPCCP is associated with the plasma membrane and internal pollen tube vesicles. Colocalization between an NaPCCPgreen fluorescent protein fusion and internalized FM4-64 suggest an association with the endosomal system. NaPCCP localization is altered in pollen tubes rejected by the self-incompatibility mechanism, but our hypothesis is that it has a general function in the transport of endocytic cargo rather than a specific function in self-incompatibility. NaPCCP represents a bifunctional protein with both phosphatidylinositol 3-phosphate- and arabinogalactan protein-binding domains. Therefore, it could function in the transport of pistil ECM proteins in the pollen tube endomembrane system.