AP-4 complex subunit sigma-1 (AP4S1)

AP-4 complex subunit sigma-1 (AP4S1) is the small, sigma-1 subunit of the heterotetrameric AP-4 adaptor protein complex. The AP-4 complex consists of two large chains (beta-4, epsilon-4), one medium chain (mu-4), and this small sigma-4 chain (AP4S1). These adaptor protein complexes are essential for sorting integral membrane proteins at key stages of the cell’s endocytic and secretory trafficking pathways. AP-4, in particular, mediates export of transmembrane cargo, including autophagy protein ATG9A, from the trans-Golgi network. Unlike other AP complexes, AP-4 is less abundant but ubiquitously expressed, supporting its specialized but critical role in all cell types. Mutations in any AP-4 subunit, including AP4S1, cause AP-4-deficiency syndrome—a hereditary spastic paraplegia with neurodevelopmental impairment. The AP-4 complex assembly is facilitated by the chaperone AAGAB, which stabilizes the AP-4 epsilon and sigma-4 subunits (AP4S1) for proper complex formation. There are no known drugs that directly target AP4S1, nor established mechanisms, biomarkers, or safety concerns relevant to therapeutic targeting.

Other names

AP4S1AP47BCLA20CLAPS4CPSQ6SPG52adaptor related protein complex 4 sigma 1 subunitSigma-4-adaptin