Adaptor protein complexes AP-4 and AP-5: new players in endosomal trafficking and progressive spastic paraplegia.

The adaptor proteins (APs) are a family of five heterotetrameric complexes with important functions in vesicle trafficking. While the roles of APs 1-3 are broadly established, comparatively little is known about AP-4 and AP-5. Current evidence suggests that AP-4 mediates TGN to endosome transport of specific cargo proteins, such as the amyloid precursor protein APP, and that it is involved in basolateral sorting in polarized cells. Furthermore, several independent studies have reported human patients with mutations in AP-4 genes. AP-4 deficiency causes severe intellectual disability and progressive spastic para- or tetraplegia, supporting an important role for AP-4 in brain function and development. The newly discovered AP-5 complex appears to be involved in endosomal dynamics; its precise localization and function are still unclear. Intriguingly, AP-5 deficiency is also associated with progressive spastic paraplegia, suggesting that AP-5, like AP-4, plays a fundamental role in neuronal development and homeostasis. The unexpected phenotypic parallels between AP-4 and AP-5 patients may in turn suggest a functional relationship of the two APs in vesicle trafficking.