Minisymposium 7: Protein turnover

Abs # 17002: Autophagy in Plants: Identification of putative homologs in Arabidopsis and rice, expression of autophagy genes in response to nutrient stress, and molecular characterization of AtATG6.

Presenter:	Laporte, Marianne , marianne.laporte@emich.edu
Authors	Laporte, Marianne  (A)   Harrison-Lowe, Nicola  (B)   Yerram, Mamatha  (A)   Olsen, Laura  (B)
Affiliations:	(A): Eastern Michigan University (B): University of Michigan

Autophagy is a regulated recycling process that enables organisms to survive during various stress conditions. During autophagy, double-membrane bound vesicles known as autophagosomes traffic cytosolic contents or organelles to the vacuole where they are broken down and reallocated to essential processes. While extensively studied in yeast and mammalian systems, this process has not been well characterized in plants. We have begun a multi-faceted investigation to understand plant autophagy. First, sequence similarity and motif analyses were used to identify putative autophagy genes in the Arabidopsis and rice genomes. Although at least 27 genes are directly involved in autophagy in yeast, homologs of only 16 are easily identified in the Arabidopsis genome. Of those 16, only 11 appear to have homologs in rice. Interestingly, some proteins, such as ATG14, that are essential for autophagy in yeast, have not been identified in Arabidopsis or rice based on sequence comparison. Second, an Arabidopsis cell-culture system was developed to determine the environmental conditions that will induce autophagy. We have used this system to identify potential molecular markers for autophagy using RT-PCR. Expression of the Arabidopsis homologs to the yeast genes ATG3, ATG6, and ATG9 increased over time in response to nitrogen deficiency. Third, we have identified a series of TDNA-insertion mutants with putative defects in predicted autophagy genes in Arabidopsis. The analysis of these lines includes biochemical, molecular, and ultrastructural characterization. Finally, we have focused on the role of AtATG6 in plant autophagy. AtATG6 is 26% identical and 46% similar to the yeast ATG6 protein. It is 31% identical and 49% similar to the human ATG6 homolog, beclin1, at the amino acid level. It aligns to the Pfam APG6 family (PF04111) and contains a coiled-coil protein interaction domain and a putative amino-terminal nuclear export signal. Mutants homozygous for a TDNA insert in AtATG6 are not viable when grown in soil, but the lines can be maintained as heterozygotes. In culture, homozygous AtATG6 mutants grow slowly and eventually flower but fail to set seed. The proteins that interact with ATG6 in plants are unknown; the homologs to the yeast genes have not been found in the Arabidopsis genome. Identification of the proteins that interact with AtATG6 may provide new insights into the role of this protein in autophagy and the process of plant autophagy in general.