8.A.203.  The Adaptor Protein 4 Chaparone (AAGAB) Family 

The adaptor protein chaperone, AAGAB, stabilizes AP-4 complex subunits (Mattera et al. 2022). Adaptor protein 4 (AP-4) is a heterotetrameric complex composed of epsilon, beta4, mu4, and sigma4 subunits that mediates export of a subset of transmembrane cargos, including autophagy protein 9A (ATG9A), from the trans-Golgi network (TGN). AP-4 has received attention because mutations in any of its subunits cause a complicated form of hereditary spastic paraplegia referred to as "AP-4-deficiency syndrome." The identification of proteins that interact with AP-4 has revealed the mechanisms of AP-4-dependent cargo sorting and distribution within the cell. Mattera et al. 2022 reported that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 epsilon and sigma4 subunits, thus promoting complex assembly. The physiological importance of these interactions is underscored by the observation that AAGAB-knockout cells exhibit reduced levels of AP-4 subunits and accumulation of ATG9A at the TGN like those in cells with mutations in AP-4-subunit genes. Thus, AP-4 assembly is not spontaneous but AAGAB-assisted, further contributing to the understanding of an adaptor protein complex that is critically involved in development of the central nervous system (Mattera et al. 2022).