Damage signals send monomeric NPR1 (green) into the nucleus (right).

Dong/Elsevier

A plant signaling protein is turned on when its intermolecular disulfide bonds are reduced and it splits into a monomeric form, say Zhonglin Mou, Weihua Fan, and Xinnian Dong (Duke University, Durham, NC). Only the monomeric form of this NPR1 protein can enter the nucleus and activate transcription.

NPR1 is made constitutively, but activated by salicylic acid (SA) as part of a general plant defense against infection. Dong initially set out to purify the NPR1 complex by gel filtration. She was puzzled that an NPR1 peak was found only with SA-treated samples, but then discovered that in the uninduced samples DTT mimicked SA: it liberated monomeric NPR1 from a complex that was too big to enter the column.

SA is produced when plants first blast away at infecting microbes with oxidants, with SA prompting various enzymes to boost oxidant production. The cells then overcompensate with antioxidants.

It is this later reducing environment that unhinges NPR1 from an oligomeric complex, say the researchers. Cysteine-substituted NPR1 mutants were constitutively monomeric and constitutively active in inducing downstream PR genes, which provide longer-term antimicrobial defenses.

The intermolecularly bonded storage form appears to be novel for signaling networks, although intramolecular disulfide formation is known to drive conformational changes that either activate transcription (by bacterial OxyR) or conceal a nuclear export signal (in yeast yAP1). ▪

Reference:

Mou, Z., et al. 2003. Cell. 113:935–944.