AP1/2 beta-mediated exocytosis of tapetum-specific transporters is required for pollen development in Arabidopsis thaliana

Arabidopsis AP1/2 beta adaptins are shared by the AP-1 and AP-2 complexes and required for pollen development by mediating the trafficking of ABCG transporters in tapetal cells. AP-1 and AP-2 adaptor protein (AP) complexes mediate clathrin-dependent trafficking at the trans-Golgi network (TGN) and the plasma membrane, respectively. Whereas AP-1 is required for trafficking to plasma membrane and vacuoles, AP-2 mediates endocytosis. These AP complexes consist of four subunits (adaptins): two large subunits (beta 1 and gamma for AP-1 and beta 2 and alpha for AP-2), a medium subunit mu, and a small subunit sigma. In general, adaptins are unique to each AP complex, with the exception of beta subunits that are shared by AP-1 and AP-2 in some invertebrates. Here, we show that the two putative Arabidopsis thaliana AP1/2 beta adaptins co-assemble with both AP-1 and AP-2 subunits and regulate exocytosis and endocytosis in root cells, consistent with their dual localization at the TGN and plasma membrane. Deletion of both beta adaptins is lethal in plants. We identified a critical role of beta adaptins in pollen wall formation and reproduction, involving the regulation of membrane trafficking in the tapetum and pollen germination. In tapetal cells, beta adaptins localize almost exclusively to the TGN and mediate exocytosis of the plasma membrane transporters such as ATP-binding cassette (ABC)G9 and ABCG16. This study highlights the essential role of AP1/2 beta adaptins in plants and their specialized roles in specific cell types.