TIR1

TIR1, short for Transport Inhibitor Response 1, is an F-box protein that functions as part of the auxin signaling pathway in plants. It is a key component of the SCF (SKP1-CUL1-F-box) ubiquitin ligase complex, acting as an auxin receptor in conjunction with other F-box proteins of the TIR1/AFB family. In Arabidopsis thaliana, TIR1 belongs to a family that includes several Auxin Signaling F-box (AFB) paralogs, collectively extending the plant’s capacity to perceive and respond to auxin.

Mechanistically, TIR1 promotes the degradation of Aux/IAA transcriptional repressors in the presence of the plant hormone auxin (such as indole-3-acetic acid). Auxin acts as a molecular glue that enhances the interaction between TIR1 and Aux/IAA proteins, enabling their ubiquitination by the SCF complex and subsequent proteasomal degradation. This derepresses AUXIN RESPONSE FACTOR (ARF) transcription factors, stimulating the transcription of auxin-responsive genes.

Structurally, TIR1 contains an F-box domain that recruits Skp1 and forms the SCF core, along with leucine-rich repeats that mediate substrate binding, including Aux/IAA proteins. TIR1 is predominantly localized in the nucleus, where auxin signaling modulates developmental processes such as root and shoot growth, organ formation, and tropic responses.

Discovery and significance: The role of TIR1 as an auxin receptor was demonstrated in the mid-2000s by studies showing that SCF(TIR1) perceives auxin and targets Aux/IAA repressors for degradation. The TIR1/AFB family is widely conserved in land plants and underpins the core regulatory module of auxin signaling.