Biogenesis of mitochondria and chloroplasts II

Question 1

transport complexes in chloroplasts

Answer 1

TOC and TIC

Question 2

receptors which recognise chloroplast transit peptides

Answer 2

Toc159, Toc33

Question 3

inner membrane targeting routes

Answer 3

stop-transfer and conservative sorting pathways

Question 4

how many pathways for delivery to the thylakoids

Answer 4

2 lumenal, 2 membrane- 4 total

Question 5

4 pathways

Answer 5

cpSec- analogous to TIC-TOC system
cpTat- powered by proton gradient, transports folded proteins
cpSRP- similar to CRP pathway in the ER, post-translational system however
spontaneous insertion- for small proteins, you may not necessarily need assistance

Question 6

differences between chloroplast and mitochondrial targeting sequences

Answer 6

actually pretty similar in terms of overall structure

overrepresentation of different amino acids in N-terminal residues

cTPs are a little longer- might help w lack of proton gradient??

cTPs are less structured and do not necessarily form the amphipathic a helixes

mitochondrial protein import receptors are also lacking in plants, so makes this problem potentially less of an issue

Question 7

when might dual targeting happen

Answer 7

when proteins are involved in stuff like DNA replication, maintenance etc and are required in both

Question 8

how does dual targeting happen

Answer 8

either twin targeting sequences- separate coding sequences for each
or ambiguous targeting sequences- signals which have intermediate characteristics

Question 9

2 hypotheses of organelle gene maintenance

Answer 9

hydrophobicity hypothesis- organellar genomes consist of genes which are v hydrophobic and therefore very annoying to keep moving between the nucleus and the organelle
co-location foe redox regulation (CORR) theory- allowing for quicker and more targetied responses to any imbalances in electron transfer systems

Question 10

issues w hydrophobicity hypothesis

Answer 10

mit and chl do import a lot of hydrophobic proteins, and not all encoded proteins are hydrphobic

Question 11

issues w CORR hypothesis

Answer 11

not all organellar genes encode these redox genes, and often these processes need nuclear genes to work properly anyway

Question 12

some other ideas for why some genes are retained

Answer 12

genetic code disparities between nuclear and other genomes

toxicity of genome products if in the nucleus

limited transfer window- need cells with multiple organelles for this to even happen, as organelle lysis is required for the transfer of genes into the nucleus