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

Dominik Ziehe  1 ,

Review

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

Dominik Ziehe et al. Photosynth Res. 2018 Dec.

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.

Keywords: Alb3; CpFtsY; CpSRP; LHCP; Thylakoid membrane; Transit complex.

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Figures

Fig. 1

LHCPs are targeted to the thylakoid membrane via the posttranslational cpSRP-dependent transport pathway. LHCPs are imported posttranslationally into chloroplasts via the TOC/TIC translocon in the outer and inner envelope membrane. After import into the stroma, the transit peptide is cleaved off and the LHCPs are forwarded to the cpSRP complex by LTD. The transit complex consisting of cpSRP43, cpSRP54, and LHCP traverses the stroma and docks to the thylakoid membrane via interaction with cpFtsY and the Alb3 insertase. Alb3/cpFtsY are associated with the cpSecY translocase, which is, however, most likely not involved in the insertion process. GTP hydrolysis catalyzed by the SRP GTPases cpSRP54 and cpFtsY drives the dissociation of protein components

Fig. 2

Scheme of protein–protein interactions within the posttranslational cpSRP transport pathway. The mature forms of the proteins involved in posttranslational LHCP transport and their functional domains are shown. Amino acid positions on the left and right correspond to the Arabidopsis thaliana proteins, except of LHCP, which refers to Lhcb1 of Pisum sativum. The diverse interaction partners and the interacting domains are indicated. Further details and references are given in the main text. a LHCP harbors three transmembrane domains (dark green, 1–3). The L18 region containing the crucial DPLG motif, which is responsible for cpSRP43 (Ank3) binding, is located between the second and third transmembrane domains. The binding region for cpSRP43 extends into LHCP’s transmembrane domain three. It is discussed if cpSRP54 binds to transmembrane domain three of LHCP. Furthermore, there is a direct interaction between Alb3’s C-terminus and LHCP. b CpSRP43 comprises three chromodomains (CD1-CD3, red) and four ankyrin repeats (Ank1-Ank4, orange). The Ank2-Ank4 region with the conserved Y204 binds the LHCP L18 peptide. The interaction with the ARRKR motif of cpSRP54 is accomplished via a twinned aromatic cage located in CD2, which is formed by six residues. A stimulating effect of Ank4 for cpSRP54/cpSRP43 complex formation was demonstrated (depicted by dashed line). CpSRP43 binds to the Alb3’s C-terminus (motifs II and IV) via CD2-3, whereby CD3 plays the major role (dashed and solid lines, respectively). c CpSRP54 is composed of a N-terminal NG domain (gray) and a C-terminal M domain (yellow) connected by a linker region (light gray). CpSRP54 binds to cpSRP43 with its C-terminal ARRKR motif (red) within the M domain. The NG domain binds to the homologous domain in cpFtsY. An acidic patch (dark shaded, E313; D314; E316; D317) next to the M domain forms an additional interaction site for cpFtsY. The M domain possibly also binds to the third transmembrane domain of LHCP. d Like cpSRP54, cpFtsY comprises a NG domain. A membrane targeting sequence (MTS, dark shaded) is located close to the N-terminus. As mentioned above cpFtsY interacts with cpSRP54 via its NG domain. Additionally, it contains a basic patch (K191; K193; K203; R204; K235; K236; K 240) as counterpart for cpSRP54’s acidic patch to provide an interaction via the complementary charged regions. e Alb3 is predicted to have five transmembrane domains which are summarized and depicted as transmembrane (TM) region (dark blue). CpSRP43 binds to a motif within the TM region, motif II and motif IV. Further data indicate that the binding to motif II and motif IV is mediated by CD1-Ank4 and CD3, respectively. The Alb3 C-terminus also binds the cpSRP54/cpFtsY complex, whereby the binding interface is probably provided by motifs II and IV. Additionally, LHCPs bind to the C-terminal region of Alb3

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