Calcium binding to the photosystem II subunit CP29

Area of Science:

• Plant molecular biology
• Photosynthesis research
• Protein biochemistry

Background:

• Chlorophyll a/b-binding protein CP29 is a light-harvesting protein in photosystem II.
• CP29 is implicated in the photoregulation of photosynthesis.
• The specific ion-binding sites within CP29 are not fully characterized.

Purpose of the Study:

• To identify the calcium (Ca2+)-binding site in CP29.
• To elucidate the role of Ca2+ in CP29 function and photoregulation.

Main Methods:

• Calcium-45 (45Ca2+) binding assays and dot blot analyses were used to confirm CP29 as a Ca2+-binding protein.
• Ytterbium (Yb3+) replacement and near-infrared Yb3+ vibronic side band fluorescence spectroscopy were employed to analyze the Ca2+-binding site.
• Structural modeling and analysis of dicyclohexylcarbodiimide (DCCD) binding were used to pinpoint the Ca2+ binding location.

Main Results:

• CP29 was confirmed as a Ca2+-binding protein through direct binding studies.
• Spectroscopic analysis of Yb3+-reconstituted CP29 revealed a single type of binding site rich in carboxylic acids, characteristic of Ca2+ binding.
• A structural model suggested two potential extracellular loops for ion binding; the lumenal loop, adjacent to Glu166, was identified as the likely Ca2+ binding site.
• DCCD binding at Glu166 inhibited Ca2+ binding, supporting the lumenal loop as the Ca2+ binding domain.

Conclusions:

• The Ca2+-binding site in CP29 is located within the lumenal loop between helices B and C, near Glu166.
• This Ca2+ binding likely plays a role in the photoregulation function of CP29 in photosystem II.