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The Rice -Amylase Glycoprotein Is Targeted from the Golgi Apparatus through the Secretory Pathway to the Plastids^[W],[OA]

^a Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
^b National Institute of Agrobiological Sciences, Ibaraki 305-8581, Japan
^c RIKEN Plant Science Center, Kanagawa 230-0045, Japan
^d Laboratory of Plant Nutrition, Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan
^e Molecular Membrane Biology Laboratory, RIKEN Advanced Science Institute, Saitama 351-0198, Japan
^f Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

⁵ Address correspondence to t.mitsui{at}agr.niigata-u.ac.jp.

The well-characterized secretory glycoprotein, rice (Oryza sativa) -amylase isoform I-1 (AmyI-1), was localized within the plastids and proved to be involved in the degradation of starch granules in the organelles of rice cells. In addition, a large portion of transiently expressed AmyI-1 fused to green fluorescent protein (AmyI-1-GFP) colocalized with a simultaneously expressed fluorescent plastid marker in onion (Allium cepa) epidermal cells. The plastid targeting of AmyI-1 was inhibited by both dominant-negative and constitutively active mutants of Arabidopsis thaliana ARF1 and Arabidopsis SAR1, which arrest endoplasmic reticulum-to-Golgi traffic. In cells expressing fluorescent trans-Golgi and plastid markers, these fluorescent markers frequently colocalized when coexpressed with AmyI-1. Three-dimensional time-lapse imaging and electron microscopy of high-pressure frozen/freeze-substituted cells demonstrated that contact of the Golgi-derived membrane vesicles with cargo and subsequent absorption into plastids occur within the cells. The transient expression of a series of C-terminal-truncated AmyI-1-GFP fusion proteins in the onion cell system showed that the region from Trp-301 to Gln-369 is necessary for plastid targeting of AmyI-1. Furthermore, the results obtained by site-directed mutations of Trp-302 and Gly-354, located on the surface and on opposite sides of the AmyI-1 protein, suggest that multiple surface regions are necessary for plastid targeting. Thus, Golgi-to-plastid traffic appears to be involved in the transport of glycoproteins to plastids and plastid targeting seems to be accomplished in a sorting signal–dependent manner.

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