Molecular mechanism of SRP-dependent light-harvesting protein transport to the thylakoid membrane in plants

Abstract

The light-harvesting chlorophyll a/b binding proteins (LHCP) belong to a large family of membrane proteins. They form the antenna complexes of photosystem I and II and function in light absorption and transfer of the excitation energy to the photosystems. As nuclear-encoded proteins, the LHCPs are imported into the chloroplast and further targeted to their final destination—the thylakoid membrane. Due to their hydrophobicity, the formation of the so-called ‘transit complex’ in the stroma is important to prevent their aggregation in this aqueous environment. The posttranslational LHCP targeting mechanism is well regulated through the interaction of various soluble and membrane-associated protein components and includes several steps: the binding of the LHCP to the heterodimeric cpSRP43/cpSRP54 complex to form the soluble transit complex; the docking of the transit complex to the SRP receptor cpFtsY and the Alb3 translocase at the membrane followed by the release and integration of the LHCP into the thylakoid membrane in a GTP-dependent manner. This review summarizes the molecular mechanisms and dynamics behind the posttranslational LHCP targeting to the thylakoid membrane of Arabidopsis thaliana.

Co-Translational Protein Folding and Sorting in Chloroplasts

“…The M domain of cpSRP54 lost the ability to bind SRP RNA [100], but gained a C-terminal extension containing the ARRKR motif, which determines the exclusive interaction between cpSRP54 and either cpSRP43 or the ribosome [97,104,105]. While the post-translational targeting pathway of cpSRP54 and cpSRP43 is relatively well understood, reviewed in [106], rather little is known about the co-translational function and the mechanism of target protein integration into thylakoid membranes. At least in Pisum sativum, binding to the ribosome is proposed to occur via the M domain interacting with the surface-exposed part of uL4c, a ribosomal protein of the large subunit ( Figure 2) [107].…”

“…Once the RNC-SRP complex docks to the thylakoid membrane via binding to the cpFtsY receptor (Figure 2), the M domain of cpSRP54 is hypothesized to be repositioned towards the peptide exit tunnel to facilitate translocation of the nascent chain through the translocon. In chloroplasts the translocon consists of a reduced Sec machinery (cpSecY/E), which interacts with the integrase Alb3 for special substrates (Figure 2) reviewed in [24,106,112,113]. As there is not much structural information available about the specific interactions between cpSRP54, the ribosome, cpFtsY or the translocon, it is still unclear when exactly GTP hydrolysis occurs, how the ribosome-nascent chain complex is exactly positioned at the translocon and how nascent polypeptides are co-translationally integrated into thylakoid membranes.…”

Functional studies of CpSRP54 in diatoms show that the mechanism of thylakoid protein insertion differs from that in plants and green algae

“…The photosynthetic apparatus of eukaryotic organisms is composed of proteins encoded by both nucleus‐ and chloroplast‐localized genes. In Viridiplantae, the nucleus‐encoded LHCPs and selected chloroplast‐encoded proteins are guided to thylakoid membranes by the chloroplast signal recognition particle (CpSRP) pathway (Kirst and Melis, 2014; Jeong et al ., 2017; Ziehe et al ., 2017; Ziehe et al ., 2018). The CpSRP54 GTPase of the CpSRP pathway has a dual function in plants and targets proteins both post‐translationally (LHCPs) and co‐translationally (chloroplast‐encoded thylakoid membrane proteins) to thylakoids in cooperation with other members of the pathway (Pilgrim et al ., 1998; Amin et al ., 1999; Rutschow et al ., 2008; Hristou et al ., 2019).…”

The BF4 and p71 antenna mutants from Chlamydomonas reinhardtii

“…The LHCP mRNAs are translated in the cytosol into pre-apoproteins which are translocated through the chloroplast envelope by the TOC/TIC translocon before being processed to their mature form and inserted, as holoproteins folded with their pigment complement, in the thylakoid membranes. Several biogenesis factors are required for completion of the LHCP journey from the nucleo-cytosol to the thylakoid membranes (for reviews see [5,6]). In Chlamydomonas, LHCP translation is controlled by the NAB1 protein, which is a repressor under redox control [7].…”

“…LTD hands them to the SRP complex comprised of two subunits cpSRP43 and cpSRP54. The LHCP-SRP complex is then targeted across the stroma to the thylakoid membranes where it docks to cpFtsY, the cpSRP complex receptor [6]. While cpFtsY interacts with cpSRP54 via their homologous GTPase domains, cpSRP43 and Alb3 interact directly via their C-terminal domains.…”