Comparative features of copper ATPases ATP7A and ATP7B heterologously expressed in COS-1 cells

ATP7A and ATP7B are P-type ATPases required for copper homeostasis and involved in the etiology of Menkes and Wilson diseases. We used heterologous expression of ATP7A or ATP7B in COS-1 cells infected with adenovirus vectors to characterize differential features pertinent to each protein expressed in the same mammalian cell type, rather than to extrinsic factors related to different cells sustaining expression. Electrophoretic analysis of the expressed protein, before and after purification, prior or subsequent to treatment with endoglycosidase, and evidenced by protein or glycoprotein staining as well as Western blotting, indicates that the ATP7A protein is glycosylated while ATP7B is not. This is consistent with the prevalence of glycosylation motifs in the ATP7A sequence, and not in ATP7B. ATP7A and ATP7B undergo copper-dependent phosphorylation by utilization of ATP, forming equal levels of an "alkali labile" phosphoenzyme intermediate that undergoes similar catalytic (P-type ATPase) turnover in both enzymes. In addition, incubation with ATP yields an "alkali stable" phosphoprotein fraction, attributed to phosphorylation of serines. Alkali stable phosphorylation occurs at lower levels in ATP7A, consistent with a different distribution of serines in the amino acid sequence. Immunostaining of COS-1 cells sustaining heterologous expression shows initial association of both ATP7A and ATP7B with Golgi and the trans-Golgi network. However, in the presence of added copper, ATP7A undergoes prevalent association with the plasma membrane while ATP7B exhibits intense trafficking with cytosolic vesicles. Glycosylation of ATP7A and phosphorylation of ATP7B apparently contribute to their different trafficking and membrane association when expressed in the same cell type.