Mechanism of Dual Targeting of the Phytochrome Signaling Component HEMERA/pTAC12 to Plastids and the Nucleus

Abstract

HEMERA (HMR) is a nuclear and plastidial dual-targeted protein. While it functions in the nucleus as a transcriptional coactivator in phytochrome signaling to regulate a distinct set of light-responsive, growth-relevant genes, in plastids it is known as pTAC12, which associates with the plastid-encoded RNA polymerase, and is essential for inducing the plastomic photosynthetic genes and initiating chloroplast biogenesis. However, the mechanism of targeting HMR to the nucleus and plastids is still poorly understood. Here, we show that HMR can be directly imported into chloroplasts through a transit peptide residing in the N-terminal 50 amino acids. Upon cleavage of the transit peptide and additional proteolytic processing, mature HMR, which begins from Lys-58, retains its biochemical properties in phytochrome signaling. Unexpectedly, expression of mature HMR failed to rescue not only the plastidial but also the nuclear defects of the hmr mutant. This is because the predicted nuclear localization signals of HMR are nonfunctional, and therefore mature HMR is unable to accumulate in either plastids or the nucleus. Surprisingly, fusing the transit peptide of the small subunit of Rubisco with mature HMR rescues both its plastidial and nuclear localization and functions. These results, combined with the observation that the nuclear form of HMR has the same reduced molecular mass as plastidial HMR, support a retrograde protein translocation mechanism in which HMR is targeted first to plastids, processed to the mature form, and then relocated to the nucleus.

Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

“…Proteins that were initially targeted to the plastids and subsequently relocated to the nucleus might have a role in retrograde signaling. This mechanism of translocation was recently confirmed for HEMERA/pTAC12, which was targeted first to plastids and, after cleavage of its transit peptide, was relocated to the nucleus [31]. Our results suggest that ANN5 is localized primarily to the nucleus and then relocates to plastids.…”

H2O2 as a Feedback Signal on Dual-Located WHIRLY1 Associates with Leaf Senescence in Arabidopsis

“…For example, several retrograde signaling molecules, such as O 2 and H 2 O 2 from plastids, evoke regulatory information to the cytosol and nucleus via downstream messengers and/or a complex signaling network [4,5]. Other plastid signal molecules, including 3 -phosphoadenosine 5Ȳ-phosphate (PAP, a phosphonucleotide) [6], methylerythritol cyclodiphosphate (MEcPP, an isoprenoid intermediate) [7], heme [8,9], and malate [10], are also reported. In other cases, a signal transduction may be trigged via the movement of proteins from organelles to the nucleus, especially those of the membrane-bound proteins or dual-targeting transcription factors.…”

Retrograde Signals Navigate the Path to Chloroplast Development

“…The newly 238 synthetized pTAC12/HMR has a plastid transit peptide and is transported to the plastids where 239 it is processed for the subsequent nuclear localization. However, the mechanisms and regulation 240 of this translocation from the plastids to the nucleus is unknown (Nevarez et al, 2017). Detailed 241 investigations of the timing of pTAC12/HMR localisation during early light response and 242 chloroplast development could confirm the exciting proposition that pTAC12/HMR might be 243 involved in the co-ordination of photosynthetic gene expression in both the nucleus and the 244 chloroplast in response to light.…”

“…The protein is essential for PEP activity and chloroplast development in the light, which is reflected in the albino phenotype of the mutant. In addition to the role of pTAC12/HMR in the plastids, the protein could potentially act as a plastid signal transducer as it has also been identified in the nucleus associated with Phy (Chen et al, 2010;Galvão et al, 2012;Nevarez et al, 2017).…”