Arabidopsis thaliana aminopeptidase P1 (APP1) functions as a rate-limiting component of auxin signaling transduction

Deepti Sanjai, Purdue University

Abstract

Arabidopsis thaliana aminopeptidase P1 (APP1) is an M24 metallopeptidase with limited proteolytic activity against X-pro, Arg, Lys, and Tyr N-terminal amino acids and prolyl cis-trans isomerase (PPIase) activity against internal prolyl linkages (Makam et al., 2009). APP1 is a soluble protein, and its enzymatic activities increase in the presence of the auxin transport inhibitor 1-naphthylphthalamic acid (Murphy et al., 2002; Murphy and Taiz, 1999b). APP1 exhibits a particularly strong affinity for peptides containing the motif GWPPVR. This affinity directed our attention to a potential role of APP1 in the TIR1-AUX/IAA signaling mechanism that regulates auxin-dependent gene transcription. Under basal conditions of the phytohormone auxin, AUX/IAA binding to Auxin Response Factor (ARF) transcription factors results in repression of auxin-dependent gene transcription (Paciorek and Friml, 2006; Tiwari et al., 2004). Auxin binding to TIR1 receptor of the SCFTIR1 E3 ubiquitin ligase complex activates ubiquitination and proteolytic degradation of short-lived AUX/IAA proteins, thus de-repressing auxin-responsive gene transcription (Dharmasiri et al., 2005; Kepinski and Leyser, 2002). Interactions of AUX/IAA proteins with TIR1/AFBs and ARFs are dependent on an interaction domain characterized by an essential GWPPVR motif (Ramos et al., 2002). Mutation of the second Pro of this motif results in loss of binding and auxin insensitivity. Auxin signal transduction mediated by this pathway has been shown to be strongly inhibited by treatment with the PPIase inhibitor juglone (Dharmasiri et al., 2003). Since TIR1-AUX/IAA interactions in plant extracts treated with auxin occur at different rates than similar assays using recombinant proteins, it has been suggested that a soluble component within the cell mediates their interactions. This led us to hypothesize that APP1 functions as a rate-limiting component of TIR1-AUX/IAA signaling and that APP1 may serve as an integration point of auxin signaling with wounding and other stress responses. In this thesis, we elucidate the role of APP1 using a combination of systems biology, molecular biology and biochemistry, and indicate that APP1 interacts with AUX/IAAs in an auxin-independent manner.

Degree

M.S.

Advisors

Murphy, Purdue University.

Subject Area

Molecular biology

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