PAP8/pTAC6 Is Part of a Nuclear Protein Complex and Displays RNA Recognition Motifs of Viral Origin

Abstract

Chloroplast biogenesis depends on a complex transcriptional program involving coordinated expression of plastid and nuclear genes. In particular, photosynthesis-associated plastid genes are expressed by the plastid-encoded polymerase (PEP) that undergoes a structural rearrangement during chloroplast formation. The prokaryotic-type core enzyme is rebuilt into a larger complex by the addition of nuclear-encoded PEP-associated proteins (PAP1 to PAP12). Among the PAPs, some have been detected in the nucleus (PAP5 and PAP8), where they could serve a nuclear function required for efficient chloroplast biogenesis. Here, we detected PAP8 in a large nuclear subcomplex that may include other subunits of the plastid-encoded RNA polymerase. We have made use of PAP8 recombinant proteins in Arabidopsis thaliana to decouple its nucleus- and chloroplast-associated functions and found hypomorphic mutants pointing at essential amino acids. While the origin of the PAP8 gene remained elusive, we have found in its sequence a micro-homologous domain located within a large structural homology with a rhinoviral RNA-dependent RNA polymerase, highlighting potential RNA recognition motifs in PAP8. PAP8 in vitro RNA binding activity suggests that this domain is functional. Hence, we propose that the acquisition of PAPs may have occurred during evolution by different routes, including lateral gene transfer.

Three-Dimensional Envelope and Subunit Interactions of the Plastid-Encoded RNA Polymerase from Sinapis alba

“…The PAPs, acting as signaling components expressed after phytochrome activation in the nucleus of angiosperms, may have been required to control PEP activity by the nucleus in order to synchronize the transcription of the photosynthesis-associated nuclear genes ( PhANGs ) and photosynthesis-associated plastid genes ( PhAPGs ) for the proper building of the photosynthetic apparatus upon first illumination. Due to their dual localization, some of the PAPs such as PAP5/HEMERA [ 46 ] and PAP8 [ 47 , 48 , 49 ] provided a potential regulatory link between the nucleus and plastids in the expression of photosynthesis genes. It remains to be solved whether their nuclear or their plastid function evolved first.…”

A Plastid RNA Polymerase-Associated Protein Is Involved in Early Chloroplast Development in Rice

“…Our data suggest that the OsPAP7 protein is mainly localized in the chloroplast. Considering Arabidopsis PAPs (such as PAP5, PAP8, and PAP7) were reported to target both chloroplast and nuclear [41], we could not exclude that OsPAP7 is also dual targeting. This needs to be confirmed by using a ∆cTP version of OsPAP7 for the localization experiment, as described previously [41].…”

New horizons in light control of plant photomorphogenesis and development

“…PAPs are 12 nuclear-encoded plastid proteins that are crucial for the formation of the PEP-complex and the proper build-up of chloroplasts (Pfannschmidt et al, 2015). However, six PAPs, including PAP5 and PAP8, contain a nuclear localization signal that permits them to enter the nucleus and to participate in PB-mediated PM control providing nucleo-plastidic regulation of chloroplast build-up (Pfannschmidt et al, 2015;Chambon et al, 2022). Interestingly, both nuclear and plastid versions of PAP5/ Hemera and PAP8/pTAC6 display similar sizes suggesting a plastid passage for removal of the chloroplast transit peptide in both protein versions (Chen et al, 2010;Liebers et al, 2020).…”