During development, plays a role in the regulation of the neddylation pathway. Appl and APP-BP1 interact antagonistically during development. Interacts (via the intracellular domain, ICD) with APP-BP1. Expressed in postmitotic neurons in the central and peripheral nervous systems. Within the nervous system, transcripts are not observed in neuroblasts, newly generated neurons and at least one class of presumed glial cells. Expressed in all developmental stages. The NPTY motif mediates the interaction with clathrin (By similarity). The clathrin-binding site is essential for its association with X11-alpha, -beta, and -gamma. The sequence specific recognition extends to peptide residues that are C-terminal to the NPXY motif. This interaction appears to be independent of phosphorylation (By similarity). Overexpression inhibits the Nedd8 conjugation pathway, disrupts the normal bristle pattern in the fly thorax, and induces apoptosis in wing imaginal disks. Belongs to the APP family. Was originally (PubMed:2494667) thought to be vnd but further analysis (PubMed:2127912) has clearly shown that it corresponds to Appl.