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.

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. FAD’s location can be seen in figure 1.

Figure 1: Ribbon diagram of a LOV domain, inside is the molecular structure of FAD.

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.

G

WC-1

1.

Promoter Transcribed region

WC-

2

WCC

3.

WC-2 WC-1

WCC

2.

4.

WCC

Figure 2: Graphical representation of WCC activation and function.

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-1’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.

PHR1

BLR Promoter Phr1

WC-

2

BLR

PHR1

Figure 3: Graphical representation of BLR activation and PHR1 function.

1. 2.

3.

4.

5.

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).

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.

FphAPfr

Red light

Far Red light

Dark conversion

Figure 4: Diagram illustrating the photointerconversion of FphA.

FphAPr

LINK TO SURVEY:

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