Received August 3, 2007
Returned for revision January 21, 2008
Accepted February 25, 2008
The Arabidopsis P4-ATPase ALA3 Localizes to the Golgi and Requires a 
-Subunit to Function in Lipid Translocation and Secretory Vesicle Formation
Lisbeth Rosager Poulsen 1, Rosa Laura López-Marqués 1*, Stephen C. McDowell 2, Juha Okkeri 3, Dirk Licht 3, Alexander Schulz 4, Thomas Pomorski 3, Jeffrey F. Harper 2, and Michael Gjedde Palmgren 1
1 Centre for Membrane Pumps in Cells and Disease–PUMPKIN, Danish National Research Foundation, University of Copenhagen, DK-1871 Frederiksberg C, Denmark; Department of Plant Biology, University of Copenhagen, DK-1871 Frederiksberg C, Denmark
2 Biochemistry Department MS200, University of Nevada, Reno, Nevada 89557
3 Humboldt-University Berlin, Faculty of Mathematics and Natural Science I, Institute of Biology, 10115 Berlin, Germany
4 Department of Plant Biology, University of Copenhagen, DK-1871 Frederiksberg C, Denmark
* To whom correspondence should be addressed. E-mail: rlo{at}life.ku.dk.
Vesicle budding in eukaryotes depends on the activity of lipid translocases (P4-ATPases) that have been implicated in generating lipid asymmetry between the two leaflets of the membrane and in inducing membrane curvature. We show that Aminophospholipid ATPase3 (ALA3), a member of the P4-ATPase subfamily in Arabidopsis thaliana, localizes to the Golgi apparatus and that mutations of ALA3 result in impaired growth of roots and shoots. The growth defect is accompanied by failure of the root cap to release border cells involved in the secretion of molecules required for efficient root interaction with the environment, and ala3 mutants are devoid of the characteristic trans-Golgi proliferation of slime vesicles containing polysaccharides and enzymes for secretion. In yeast complementation experiments, ALA3 function requires interaction with members of a novel family of plant membrane-bound proteins, ALIS1 to ALIS5 (for ALA-Interacting Subunit), and in this host ALA3 and ALIS1 show strong affinity for each other. In planta, ALIS1, like ALA3, localizes to Golgi-like structures and is expressed in root peripheral columella cells. We propose that the ALIS1 protein is a 
-subunit of ALA3 and that this protein complex forms an important part of the Golgi machinery required for secretory processes during plant development.
