Subunit of clathrin-associated adaptor protein complex 1 that plays a role in protein sorting at the trans-Golgi network and early endosomes (TGN/EE). The AP complexes mediate the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. Required for KNOLLE localization at the cell plate to mediate cytokinesis. Functions redundantly with AP1M1 in multiple post-Golgi trafficking pathways leading from the TGN to the vacuole, the plasma membrane, and the cell-division plane. Adaptor protein complex 1 (AP-1) is a heterotetramer composed of two large adaptins (gamma-type subunit and beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type subunit). Ubiquitous. Short pollen tube growth and failure to exit the style (PubMed:15514068). Compromised cytokinesis due to the mislocalization of the KNOLLE syntaxin (PubMed:23543752, PubMed:23733933). Full spectrum of growth defects, suggestive of compromised auxin signaling and of defective RLK signaling. Cell morphogenesis was also disturbed (PubMed:23766365). Impaired pollen function and growth retardation phenotype (PubMed:23733933). Belongs to the adaptor complexes medium subunit family.