Protein domains required for formation of stable monomeric Lhca1- and Lhca4-complexes

The peripheral light-harvesting complex of Photosystem I consists of two subpopulations, LHCI-680 and LHCI-730. The latter is composed of the two apoproteins Lhca1 and Lhca4. Recently, reconstitution of monomeric LHCI using bacterially overexpressed Lhca1 or Lhca4 was achieved. In order to obtain insight into the structure requirements for formation of monomeric light-harvesting complexes, we produced a series of N- and C-terminal deletion mutants and used the overexpressed proteins for reconstitution experiments. We found the entire extrinsic N-terminal region dispensable for monomer formation in Lhca1 and Lhca4. Also at the C-terminus, both subunits revealed similarity since all amino acids up to the end of the fourth helix could be removed without abolishing monomer formation. In connection with former corresponding results for Lhcb1, the dispensability of these regions appears to be a general feature in LHC-formation. In LHCI, however, a stabilising effect can be ascribed to these regions since the yield of complexes was decreased. In the majority of the mutant LHCI versions no effect on pigment binding was detected. However, in the LHC with the most extensively N-terminally truncated mutant of Lhca4 a dramatic shift in the 77 K fluorescence emission to shorter wavelengths was observed. This suggests that chlorophylls involved in long wavelength fluorescence emission are located in the chlorophyll array located towards the stromal face of the thylakoid membrane assuming a pigment arrangement corresponding to that in LHCII and CP29.