Poster: Photosynthesis

Abs # 336: Functional analysis of amino acid residues affecting affinity to Chls a or b in a Class II water-soluble chlorophyll protein as assayed by site-directed mutagenesis.

Presenter:	Satoh, Hiroyuki , hsatoh@biomol.sci.toho-u.ac.jp
Authors	Satoh, Hiroyuki  (A)   Zanma, Aya  (A)   Wakabayashi, Maiko  (A)   Yoshikawa, Mami  (A)   Uchida, Akira  (A)   Nakayama, Katsumi  (A)   Okada, Mitsumasa  (A)
Affiliations:	(A): Dept. of Biomol. Sci., Toho University

Water-soluble Chl proteins (WSCPs) can be categorized into two classes; a photoconvertible, Chenopodium-type (Class I) and a non-photoconvertible, Brassica-type (Class II). The absorption spectrum of a Class I WSCP changes drastically with light illumination, while that of a Class II WSCP does not. Although these WSCPs can be detected in several plants species, their physiological roles are still uncertain. By X-ray crystallographic analysis, the axial ligand of the Mg atom in Chl, and the molecular environment of amino acid residues affecting its affinity to Chls a or b have almost been elucidated. An amino acid residue for Mg-axial ligand is highly conserved among the Class II WSCPs cloned to date and is thought to play an important role in their Chl-binding activity. To clarify its structural and functional roles, we mutated this residue into alanine, glutamine, and histidine using PCR-based site-directed mutagenesis. The alanine mutant exhibited even higher Chl-binding activity than that of the wild-type, indicating that this residue does not necessarily have to be the original residue, while the glutamine mutant possessed much lower activity. Glutamine residues are larger than alanine residues and therefore may cause steric hindrance of Chl binding. In addition, we mutated four amino acid residues that may affect the binding affinity to Chls a or b. We are currently analyzing the changes in binding affinity to Chl, and the results will be presented.