The final story on the ALA3/ALIS1 complex.

Lisbeth R. Poulsen^a, Rosa L. López-Marqués^a, Alexander Schultz^a, Stephen C. McDowell^b, Juha Okkeri^c, Dirk Licht^c, Thomas Pomorski^c,  Jeffrey F. Harper^b, and Michael G. Palmgren^a,1

^aCentre for Membrane Pumps in Cells and Disease - PUMPKIN, Danish National Research Foundation, Department of Plant Biology, University of Copenhagen, DK-1871 Frederiksberg C, Denmark; ^bBiochemistry Department MS200, University of Nevada Reno, NV 89557, USA; ^cHumboldt-University Berlin, Faculty of Mathematics and Natural Science I, Institute of Biology, 10115 Berlin, Germany.

        In Arabidopsis 12 P₄-ATPase isoforms have been identified, named ALA1-12 for Aminophospholipid ATPase (Axelsen and Palmgren, 2001). So far, only one member of this family (ALA1) has been partially characterized and suggested to have a physiological relevance in chilling tolerance (Gomés et al., 2000).     

        Through a database search we have previously identified five Cdc50p/Lem3p homologues in Arabidopsis (ALIS1-5 for ALA Interacting Subunit)..We investigated the capacity of ALA3, alone and in combination with expressed ALIS proteins, to functionally complement a battery of yeast mutants carrying deletions in endogenous P₄-ATPases. Our results indicated that ALIS1 functions as a true ß-subunit for the Arabidopsis putative flippase ALA3, being required for ATP-dependent phospholipid transport and for genetic complementation of the yeast P₄-ATPase gene Drs2, which is involved in vesicle budding from the late Golgi.

        Together with ALIS1, we have found that ALA3, localises to the Golgi apparatus in plant cells. Expression of a ß-glucuronidase (GUS) gene under the control of the promoter for either ALIS1 or ALA3 showed a high expression level for both promoters at the columella root cap. Two independent lines of ala3 mutant were phenotypically analyzed. When grown under 24 h light conditions, mutant plants are significantly smaller than control wild type plants. A decrease in root growth is related to the inability of ala3 plants to release border-like cells from root caps that are damaged during root penetration of the growth medium. Electron micrographs show cellular defects in cells of the root cap of mutant plants related to the formation of vesicles in the trans side of the Golgi apparatus. We propose that the flippase activity of the ALA3/ALIS1 complex in the Golgi is required for important secretory processes involved in shedding of damaged root cap cells.

Axelsen KB, Palmgren MG (2001) Plant Physiol. 126:696-706.

Gomes E, Jakobsen MK, Axelsen KB, Geisler M, Palmgren MG (2000) Plant Cell 12:2441-2454.

Pomorski T, Menon AK (2006) Cell Mol Life Sci. 63:2908-2921.