Lecture 7: The Apicoplast Flashcards
What is the apicoplast?
a non-photosynthetic plastid present in apicocomplexan parasites
how many membranes does the apicoplast have and what are they?
four (unusual bc plastid generally only have two membranes)
- double membrane of chloroplast
- plasma membrane of algal prey
- food vacuole membrane
The organisms belonging to the phylum apicocomplexa are nearly all what?
parasite and cause major disease (important in humans and livestock as well - drug targets)
Describe the discovery of the apicoplast?
mitochondrial DNA of duck malarial parasite was isolated
- pulled out a circular cross shaped structured which is classical of inverted sequence of repeats often found in plastids
- dismissed and assumed to be mtDNA
- but then it was found that a linear DNA molecule was found that encoded mt genes = the actual mtDNA
- sequence of the 35 Kb circular DNA from P. falciparum proved that the genome had prokaryotic ancestry but the genes had closer ancestry to plastids of plants and algae
Upon sequencing of the 35 Kb circular DNA found in the parasites, what three things were revealed?
- it contained genes normally found in the chloroplast genome but no mitochondrial genes
- no genes for photosynthesis
- inverted sequence repeats (palindromic rRNA) characteristic of chloroplast genomes
How was the normal location of the 35Kb DNA circle in the cell identified?
Already sequenced so could make a probe for a specific sequence
- T. gondii cells fixed and all DNA denatured
- hybridised with digoxigenin labelled probes for the 35Kb fragment
- detect digoxigenin with gold bound probes and visualised with EM
Describe the evolution of the apicoplast?
The apicoplexan ancestor ate a plant:
- 1st endosymbiosis = organism engulfed cyanobacterium but not digested to form an alga with double membrane chloroplast
- secondary endosymbiosis = secondary host (apicomplexa ancestor) engulfed the alga but not digested
What type of alga was engulfed during the secondary endosymbiotic event?
red alga
How did the apicoplast integrate into the parasite?
transport nutrients (fetch)
- triosephosphate/phosphate transporter (TPT) - sugar transporters that in plant plastids transported sugars from photosynthesising plastid to cytosol but not the apicoplast is not photosynthetic, the TPTs transport sugars from cytosol into plastid.
lock in (stay)
- loss of genetic autonomy by gene transfer - take essential survival genes and integrate into DNA of parasite so apicoplast relies on parasite for survival
control division (stop that now)
- needs to divide equally when the cell divides to allow evolution to occur and be passed on
- apicoplasts have a physical connection between their genome and the centrioles in the cytoplasm which ensures crucial partitioning of the organelle genome into daughter cells
What is the difference in the location of the TPTs in plant plastids than the apicoplast?
TPT in plant plastids are located on the inner membrane of the double membrane organelle
Where as the TPTs of apicoplast are on the surface/outer membrane
- the outer membrane links to the ER and that’s where the TPTs have been added on the surface
How are the protein products of apicoplast genes in parasite DNA targeted to the apicoplast and cross four membranes?
- proteins made have the mature protein domain, a plant-like transit peptide, and a cleavable signal peptide that target it to the ER
- translocation across three apicoplast membranes
- maturation of the aplicoplast protein by cleavage of the transit peptide
What is the function of the apicoplast?
How was the apicoplast demonstrated to be essential?
Pharmacologically
- ciprofloxacin inhibits chloroplast but not mitochondrial DNA gyrases
–> treatment if intracellular Toxoplasma with ciprofloxacin results in inhibition of apicoplst genome replication
–> treatment causes delayed death (massive reduce in survival after on second division) showing apicoplast is essential for survival of the parasite
Describe and explain the kinetics of parasite death following ciprofloxacin treatment
No real difference in survival between treated and untreated parasites following the first cell division
but massive drop in survival of treated parasites on second cell division
Explanation:
even though the ciprofloxacin inhibited apicoplast genome division but the proteins had still been made and were present in the apicoplast during the first division
- but when the cell divides, the proteins are massively reduced and no more can be made because the apicoplast DNA has been destroyed
What are four functions believed to be of the apicoplast?
DOXP pathway in isoprenoid synthesis (lipids important for electron transport and protein glycosylation) - this pathway is unique to eukaryotic pathway (drug target)
Haem synthesis (important prosthetic group on many proteins such as cytochromes) - cannot recycle hosts haem bc they’re not always blood-borne
–> this is a complete pathway shared between mitochondria and apicoplasts - closely associated (lock in mechanism for function)
Fe-S cluster synthesis via the SUF pathway (important in oxidation/reduction, enzymes)
Fatty acid biosynthesis pathway (uses unique enzyme system FASII - different to the typical eukaryotic pathway - drug target)
