AP4S1

AP-4 complex subunit sigma-1 is a protein that in humans is encoded by the "AP4S1" gene.
Function.
The heterotetrameric adaptor protein (AP) complexes sort integral membrane proteins at various stages of the endocytic and secretory pathways. AP4 is composed of 2 large chains, beta-4 (AP4B1) and epsilon-4 (AP4E1), a medium chain, mu-4 (AP4M1), and a small chain, sigma-4 (AP4S1, this gene).
Clinical relevance.
It is currently hypothesized that AP4-complex-mediated trafficking plays a crucial role in brain development and functioning.