A total of 45 genes encoding for P-type ATPases have been identified in the complete genome sequence of Arabidopsis. Thus, this plant harbors a primary transport capability not seen in any other eukaryotic organism sequenced so far. The sequences group in all five subfamilies of P-type ATPases. The most prominent subfamilies are P1B ATPases (heavy metal pumps; seven members), P2A and P2B ATPases (Ca2+ pumps; 14 in total), P3A ATPases (plasma membrane H+ pumps; 12 members including a truncated pump, which might represent a pseudogene or an ATPase-like protein with an alternative function), and P4 ATPases (12 members). P4 ATPases have been implicated in aminophosholipid flipping but it is not known whether this is a direct or an indirect effect of pump activity. Despite this apparent plethora of pumps, Arabidopsis appears to be lacking Na+ pumps and secretory pathway (PMR1-like) Ca2+-ATPases. A cluster of Arabidopsis heavy metal pumps resembles bacterial Zn2+/Co2+/Cd2+/Pb2+ transporters. Two members of the cluster have extended C termini containing putative heavy metal binding motifs. The complete inventory of P-type ATPases in Arabidopsis is an important starting point for reverse genetic and physiological approaches aiming at elucidating the biological significance of these pumps
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