Structural Basis for Specific Substrate Recognition by the Chloroplast Signal Recognition Particle Protein cpSRP43
Katharina F. Stengel,1*
Iris Holdermann,1*
Peter Cain,2
Colin Robinson,2
Klemens Wild,1
Irmgard Sinning1
Secretory and membrane proteins carry amino-terminal signal
sequences that, in cotranslational targeting, are recognized
by the signal recognition particle protein SRP54 without sequence
specificity. The most abundant membrane proteins on Earth are
the light-harvesting chlorophyll a/b binding proteins (LHCPs).
They are synthesized in the cytoplasm, imported into the chloroplast,
and posttranslationally targeted to the thylakoid membrane by
cpSRP, a heterodimer formed by cpSRP54 and cpSRP43. We present
the 1.5 angstrom crystal structure of cpSRP43 characterized
by a unique arrangement of chromodomains and ankyrin repeats.
The overall shape and charge distribution of cpSRP43 resembles
the SRP RNA, which is absent in chloroplasts. The complex with
the internal signal sequence of LHCPs reveals that cpSRP43 specifically
recognizes a DPLG peptide motif. We describe how cpSPR43 adapts
the universally conserved SRP system to posttranslational targeting
and insertion of the LHCP family of membrane proteins.
2 Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK.
* These authors contributed equally to this work.
To whom correspondence should be addressed. E-mail: irmi.sinning{at}bzh.uni-heidelberg.de
