Minisymposium 25: Protein Targeting

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Abs # M2504: Phototropin1:subcellular localization and BL-induced movement

Presenter:	Eisinger, William R.       Contact Presenter
Authors	Eisinger, William R. (A)   Briggs, Winslow  (B)   Ehrhardt, David  (B)   Wan, Linglang  (C)   Baluska, Frantisek  (A)
Affiliations:	(A): Santa Clara University (B): Carnegie Institution Department of Plant Biology (C): University of Bonn

The blue light receptor photropin 1 (phot1) is a flavin-binding light-activated protein kinase. Although the activation of the C-terminal kinase domain has been characterized, the signal transduction pathway between perception of light by phot1 and phototropic responses remains unresolved. Phot1-GFP fusion protein is expressed in most cell types in shoots and roots of etiolated Arabidopsis seedlings (Sakamoto et al., 2002). Phot1 is known to be localized to the plasma membrane (PM) and expressed most strongly in cortical cells in apical regions. We report that within individual cortical cells phot1-GFP is mostly at the PM adjacent to other cortical cells. The latter is most prominent on the PM associated with end walls of neighboring cells in the cortex. Less expression is seen at the PM of epidermal cells. Blue light irradiation induces a transition of phot1-GFP fluorescence from a diffuse to a punctuate pattern at the PM. Accompanying this transition some fluorescence also appears in the cytoplasm. These changes are observed within minutes in several cell types in both shoot and root tissues. This appears to be a blue light response since equivalent red or green light irradiation did not induce this relocalization. The degree of relocalization is a function of blue light intensity and/or exposure time. Phot1 returns to the PM in a diffuse pattern when plants are returned to darkness for an hour or longer. Experiments with brefeldin A (inhibits protein secretion), latrunculin B (inhibits actin polymerization), FM4-64 (fluorescent PM marker), and cycloheximide suggest that blue light induces a fraction of phot1 to move to endosomal vesicles from the PM. We propose that dynamic phot1 relocalization may be involved with signal transduction for phototropism.