Cation-mediated regulation of excitation energy distribution in chloroplasts lacking organized Photosystem II complexes

Kenneth Leto, Charles Arntzen

Research output: Contribution to journalArticle

20 Citations (Scopus)

Abstract

Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf*-3 contain normal amounts of the light-harvesting chlorophyll a b pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a 'free' state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf*-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf*-3 and wild-type seedlings exhibit a similar Mg2+-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg2+ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf*-3 and wild-type seedlings show a similar Mg2+ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg2+ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg2+ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf*-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf*-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg2+-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.

Original languageEnglish (US)
Pages (from-to)107-117
Number of pages11
JournalBBA - Bioenergetics
Volume637
Issue number1
DOIs
StatePublished - Aug 12 1981
Externally publishedYes

Fingerprint

Thylakoids
Photosystem II Protein Complex
Excitation energy
Chloroplasts
Cations
Photosystem I Protein Complex
Membranes
Trypsin
Fluorescence
Association reactions
Proteolysis
Digitonin
Fractionation
Quantum efficiency
Seedlings
Pigments
Light scattering
Light
Wavelength
Zea mays

Keywords

  • (Chloroplast)
  • Cation effect
  • Chlorophyll-protein complex
  • Excitation energy distribution
  • Photosystem II mutant

ASJC Scopus subject areas

  • Biophysics

Cite this

Cation-mediated regulation of excitation energy distribution in chloroplasts lacking organized Photosystem II complexes. / Leto, Kenneth; Arntzen, Charles.

In: BBA - Bioenergetics, Vol. 637, No. 1, 12.08.1981, p. 107-117.

Research output: Contribution to journalArticle

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AU - Arntzen, Charles

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N2 - Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf*-3 contain normal amounts of the light-harvesting chlorophyll a b pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a 'free' state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf*-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf*-3 and wild-type seedlings exhibit a similar Mg2+-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg2+ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf*-3 and wild-type seedlings show a similar Mg2+ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg2+ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg2+ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf*-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf*-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg2+-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.

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