The human P5-ATPase ATP13a2 is not a Ca2+ transporting pump

DE TEZANOS PINTO, FELICITAS Y ADAMO, HUGO P.

Los Cocos, Córdoba, Argentina

Congreso; XXXVIII Reunión Anual de la Sociedad Argentina de Biofísica; 2009

Sociedad Argentina de Biofísica

The P-type ion pumps are membrane transporters energized by hydrolysis of ATP with a wide range of specificities for positively charged ions. Based on their conserved core sequences they have been classified into five subfamilies termed P1-P5 or type I-V. The most poorly characterized P-type ATPases are those of the P5 subfamily, which are expressed only in eukaryotes. Five genes named ATP13a1-ATP13a5 that belong to this group of P5-ATPases have been identified in humans. Mutations of the human gene ATP13a2 was found to underlie an autosomal recessive form of early-onset parkinsonism with pyramidal degeneration and dementia (1). The ion transported by the ATP13a2 pump is not known. However several studies of the yeast orthologue ATP13A1 (COD1P) in Saccharomyces cervisiae have shown that COD1P plays an important role in maintaining cellular calcium homeostasis in the endoplasmic reticulum (ER) and have suggested that the P5-ATPases are Ca2+ transporters (2). In order to evaluate if Ca2+ is the ion transported by this pump, chinese hamster ovarian (CHO) cells were transfected with the pcDNA3.1 expression vector carrying the ATP13a2 cDNA, and the transfectants were selected by their resistance to the antibiotic G418. Stable clones were identified and western blot assays showed the successful expression of ATP13a2 protein in microsomal membranes. The ATP dependent 45Ca2+ transport into microsomal vesicles was measured at a high concentration of free Ca2+ of 10 mM. In these conditions the transport activity by microsomes expressing ATP13A2 was similar to that of microsomes from CHO control cells. This result is in favor of the idea that the ATP13a2 is not an ATP powered Ca2+ transporter.