Chloroplast signal recognition particle (cpSRP) and ALB3: New insights into structure and function in LHCP targeting

Abstract

Recently it was shown that chloroplasts contain a specialized type of SRP (cpSRP) that contains an evolutionary conserved 54 kD subunit (cpSRP54), but differs from cytoplasmic SRPs, as it contains a novel 43 kD subunit (cpSRP43), and lacks RNA. CpSRP is also distinctive in its ability to interact with its substrate LHCP post-translationally to form a cpSRP/LHCP transit complex, which targets LHCP to the thylakoid membrane. At the thylakoid membrane LHCP interacts with ALB3, an integral membrane protein required for LHCP integration. Previous studies showed that transit complex formation is mediated through binding of the L18-domain of LHCP to cpSRP43. CpSRP43 is characterized by a four ankyrin repeat-domain at the N-terminus and two chromodomains at the C-terminus. We used the yeast two-hybrid system and in vitro binding assays to analyse the function of different domains of cpSRP43 in protein complex formation. We demonstrate that the first ankyrin repeat binds to the L18-domain of LHCP whereas the third and fourth ankyrin repeats are involved in dimerization of cpSRP43. We show further that the interaction of cpSRP43 with cpSRP54 is mediated via binding of the methione rich domain (M-domain) of cpSRP54 to the C-terminal located chromodomains of cpSRP43. Both chromodomains contain essential elements for binding cpSRP54 indicating that the closely spaced chromodomains together create a single binding site for cpSRP54. In addition our data demonstrate that the interaction of cpSRP54 with the chromodomains of cpSRP43 is enhanced indirectly by the dimerization motif of cpSRP43.