This family is represented by the multipass membrane protein XK, which may be involved in sodium-dependent transport of neutral amino acids or oligopeptides. It forms a heterodimer with Kell. In humans, Kell is an 93kDa type II membrane glycoprotein with endothelin-3-converting enzyme activity that is linked by a single disulphide bond to XK, that spans the membrane ten times. An absence of XK leads to clinical symptoms termed the McLeod syndrome , an X-linked multi-system disorder characterised by late onset abnormalities in the neuromuscular and hematopoietic systems [, ]. It has been shown that XK recruits the lipid transfer protein VPS13A from lipid droplets to the endoplasmic reticulum (ER) membrane and dysregulation of a VPS13A-XK complex is the common basis for Chorea-Acanthocytosis and McLeod Syndrome []. The human and mouse Xk-related (Xkr) families consist of nine and eight members, respectively, and mutations or variations in the gene of Xkr members are associated to human diseases. Xkr8 and other members of the family promote phosphatidylserine exposure in apoptotic cells [, , ].
