Plant Cell Advance Online Publication
Published on January 26, 2007; 10.1105/tpc.106.046631
OPEN ACCESS ARTICLE
Received August 17, 2006
Returned for revision October 30, 2006
Accepted December 19, 2006
Arabidopsis KAM2/GRV2 Is Required for Proper Endosome Formation and Functions in Vacuolar Sorting and Determination of the Embryo Growth Axis
Kentaro Tamura 1, Hideyuki Takahashi 1, Tadashi Kunieda 1, Kentaro Fuji 1, Tomoo Shimada 1, and Ikuko Hara-Nishimura 1*
1 Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
* To whom correspondence should be addressed. E-mail: ihnishi{at}gr.bot.kyoto-u.ac.jp.
We isolated an Arabidopsis thaliana mutant, katamari2 (kam2), that has a defect in the organization of endomembranes. This mutant had deformed endosomes and formed abnormally large aggregates with various organelles. Map-based cloning revealed that kam2 is allelic to gravitropism defective 2 (grv2). The KAM2/GRV2 gene encodes a homolog of a DnaJ domain-containing RECEPTOR-MEDIATED ENDOCYTOSIS-8, which is considered to play a vital role in the endocytotic pathway from the plasma membrane to lysosomes in animal cells. Immunofluorescent staining showed that KAM2/GRV2 protein localizes on punctate structures, which did not merge with any markers for Golgi, trans-Golgi network, endosomes, or prevacuolar compartments. KAM2/GRV2, which does not have a predicted transmembrane domain, was peripherally associated with the membrane surface of uncharacterized compartments. KAM2/GRV2 was expressed at the early to middle stages of seed maturation. We found kam2 mis-sorted seed storage proteins by secreting them from cells, indicating that KAM2/GRV2 is involved in the transport of the proteins into protein storage vacuoles. kam2 had another defect in embryogenesis. Half of the developing kam2-1 cotyledons grew into the opposite space of the seeds before the walking stick-shaped embryo stage. Our findings suggest that KAM2/GRV2 is required for proper formation of the endosomes involving protein trafficking to the vacuoles and determination of growth axis of the embryo.
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