A GUIDE TO FUNGAL PHOTOPIGMENTS
NOTES FROM AUTHOR: This resource is designed to give an overview of some of the photopigments found in fungi. It aims to provide a brief introduction to the different types of photopigments and the roles that they undertake in the cellular environment.
INTRODUCTION: WHAT ARE PHOTOPIGMENTS? Photopigments are proteins found ubiquitously in the biological world. Their functions range from photoreception to photosynthetic roles. Photosynthetic pigments are commonly found in plants, they are responsible for the conversion of light into chemical energy, the cornerstone of photosynthesis. Photoreceptor proteins are characterised by undergoing conformational changes when a photon is absorbed. It is common for a photoreceptor protein to only respond to light of a certain wavelength. The conformational changes cause downstream effects in the cellular environment.
WHAT IS THEIR ROLE IN FUNGI? It is obvious as to why animals need to sense light, to observe and react to their ever changing environment. The need for fungi is less clear as they are not autotrophs and they do not seem to move around their environment. These pigments coordinate a number of crucial functions in fungi such as:
Sexual and asexual development Entrainment and maintenance of the circadian clock DNA lesion repair Phototaxis
WHY ARE THEY IMPORTANT? Without the ability to entrain the circadian clock, timed cellular mechanics would enter a state of disarray. Crucial functions such as cell division are tightly regulated by circadian oscillators and with no regulation cell proliferation would spiral out of control. If replication occurs during the day it runs a greater risk of being damaged by UV radiation, circadian rhythm controlled replication minimises the hazard of UV radiation. Light helps cells sense their environment but light from the UV spectrum is extremely damaging to cellular life. The UV radiation causes lesions to form in the DNA causing mutation, certain photopigments such as photolyases repair this damage, saving the cell.
Figure 1: Ribbon diagram of a LOV domain, inside is the molecular structure of FAD.
LIGHT OXYGEN VOLTAGE (LOV) Domain Photopigments: Examples: WHITE COLLAR proteins (WC), VIVID (VVD), found in Neurospora crassa. Blue Light Regulator (BLR), found in Trichoderma atroviride. Light response A/B (LreA/B), found in Aspergillus nidulans. Chromophore: FAD Activated by: Blue Light Role: Photoadaptation and Circadian Rhythms WC is a crucial photopigment found in N. crassa, putative WC proteins such as LreA are found in other species of fungi. Chromophore: Flavin adenine dinucleotide (FAD) undergoes photoisomerization creating a flavin-cysteinyl adduct at the cysteine amino acid of the LOV domain. FADâ€™s location can be seen in figure 1.
WC Mechanism: The following is represented in figure 2; 1. Blue light causes photoisomerization FAD. 2. Change in FAD creates conformational changes in WC-1. WC-1 can now bind to WC-2 (similar structure to WC-1 but with no LOV domain) creating a white collar complex (WCC). 3. WCC binds to the promoter of certain genes allowing for their transcription. 4. WCC dissociates from promoter and is phosphorylated by an unknown protein. Figure 2: Graphical representation of WCC activation and function.
3. WCC WC-
Photoadaptation: VIVID (VVD) is a WCC-dependant protein that is responsible for photoadaptation. VVD levels increase as light intensity increases. VVD down regulates WCC activity by preventing the dimerization of WCC. The rate that VVD inhibits the WCC increases with light intensity. Overall VVD reduces the effect of WCC in prolonged exposure to light.
Circadian Rhythm Maintenance: WCC induces the transcription of genes that are involved with the circadian clock. Entrainment is the process in which the circadian clock is set at the start of the day. The start of WCC activity, due to light, entrains the clock. However WCC is extremely sensitive to light, it can even be activated by moonlight. VVD prevents this night-time activation.
VVD’s synthesis is light dependant. VVD accumulates during the day and is present during the night. VVD neutralises WCC activity at low light levels. This inhibition is removed at greater light intensities, e.g. sunrise. WCC is able to induce the transcription of genes.
OPSIN: Example: Neurospora Opsin-1 (NOP-1). Found in Neurospora crassa Chromophore: Retinal Activated by: Blue Light Role: Unknown. Chromophore: The chromophore, retinal, is covalently bound to the lysine residue on the seventh transmembrane domain. Photoisomerized from all-trans retinal to 13-cis retinal. This alteration causes conformational changes in NOP-1 structure. Mechanism and function: NOP-1 is a Type I similar in structure to Type II rhodopsin which is found in the mammalian eye to sense light. Unlike Type II, NOP-1 shows no ion pump activity and has a relatively slow photocycle. A photocycle is the process of change from resting, active, inactive, to rest again. These properties suggest that NOP-1 plays a modulatory role yet nop-1 knockout has revealed no obvious phenotype. However nop-1 knockout with the combination of CRY-1 and PHY-1 knockouts have shown to alter WCC controlled expression of CON-10, a protein involved with spore development. NOP-1â€™s (definitive) role is unknown.
PHOTOLYASE: Example: PHR1. Found in Trichoderma atroviride Chromophore: FAD and Methlytetrahydrofolate (MTHF) Activated by: Blue / UV Light Role: DNA Lesion Repair UV radiation has a damaging effect on DNA the result of which is clearly visible in humans in the form of skin cancer. This damage occurs by the UV light creating mutagenic dimers most commonly between two thymine bases. The radiation will cause one of two lesions; Cyclobutane Pyrimidine Dimer (CPD). 6-4 Pyrimidine-Pyrimidone dimer (6-4 PP). CPD formation is more common but 6-4 PP formations are far more mutagenic. These lesions can cause substitution and deletion mutations along with the potential of blocking transcription resulting in truncated proteins. These mutations happen on a regular basis and consequently need a mechanism to combat them. Such a mechanism is the use of a photolyase such as Phr1. Photolyases’ main role is to reverse the effects of the UV radiation. This process is summarised in figure 3. Mechanism and function: 1. BLR (T. atroviride WC homolog) is activated by blue/UV light. 2. BLR promotes transcription of phr1. 3. Phr1 moves to lesion site. 4. Phr1 absorbs UV photon and injects it into lesion site. 5. Lesion is reversed and DNA returns to normal state.
Figure 3: Graphical representation of BLR activation and PHR1 function.
PHYTOCHROME: Example: Fungal Phytochrome A (FphA). Found in Aspergillus nidulans Chromophore: Unknown Activated by: Red / Far Red Light Role: Sexual and asexual development Chromophore: FphA is a red/far red sensing photopigment believed to bind a chromophore similar to that of Biliverdin IXα (BV). BV has been used experimentally as FphA’s chromophore but it is not its native molecule because; A. nidulans lacks machinery to manufacture BV. Absorption spectrum of light does not match wild type spectrum. Function and Mechanism: FphA contains a Histidine Kinase (HK) output domain. HKs are involved with signal transduction. FphA alternates between two photointerconvertible states Pr and Pfr, the latter being the light activated state. Pfr allows for red light dependant HK activity, characterised by the suppression of sexual development and the promotion of asexual development. FphA will return to the inhibitory Pr state through darkness and/or far red light stimulation (see figure 4).
Figure 4: Diagram illustrating the photointerconversion of FphA.
FphAPfr Far Red light
Blue light interaction: FphA interacts with LreA and LreB, which form a WCC homolog. This Lre complex suppresses asexual development, counteracting FphA. This interaction fine tunes the developmental process. Paradoxically white light induces greater asexual development. It is not wholly understood as to why this happens.
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