Synaptonemal Complex

Question 1

meiosis I basics

Answer 1

segregation of homologues
chromosomes find each other
cross over only thing holding them together until segregation - prevent aneuploidy

Question 2

beginning of formation of the crossovers

Answer 2

formed by recombindation

> induce dsBreaks by cleaving backbone
need to repaor
repaired by HR
in meiosis - using sister for this is partially blocked
so have to use homologous chromosome - search nucleus for correct seq on homologue

Question 3

mitotic chromosome tethering to INM

Answer 3

in order to find homologue
>mitotic chromatin becomes tethered to INM at telomeres - form U-shape
>bound by LINC complex - connects them through NE to MT network
>can move chromosomes via this network
>Rapid Prophase Movements - aids in finding right homologous partner sequence

Question 4

when homologues find each other

Answer 4

recombination intermediates between homologues
tethering them at points along their sequence

synaptonemal complex can form
takes chromosomes with loose alignments at recombination sites and zips up to align them

SC required to resolve these recombination intermediates to successfully complete recombination
-most resolved in non-crossover manner
-only get one crossover per arm (usually)
-crossovers seem to inhibit formation of other ones - mediated by SC

Question 5

structure of meiotic chromosomes

Answer 5

thick chromosome axes where homologues meet
fuzzy loops of chromatin around them

Question 6

mammalian synaptonemal complex proteins

Answer 6

SYCP1:
>main transverse filament protein
>connects central and lateral filaments together

chromosome axis (either side of central):
>SYCP2,3

central filament:
>SYCE1-3
>SIX6OS1
>TEX12

all of these essential to meiosis
KO = SC fails to form
=dsBreaks fail to repair
=no resolution of recombination intermediates
=infertile

Question 7

SC self asembly

Answer 7

consists of 3 self assembling systems
constructs whole self assembling structure
-lateral element (SYCP3)
-Central element (SYCE2-TEX12)
-transverse filaments (SYCP1-SYCE3)

Question 8

SYCP3 and lateral element structure

Answer 8

SYCP3
>structural core
>N-terminal disordered region (binding other proteins)

forms banded structures on EM w 23nm spacing

last 6 AAs of SYCP c-terminal tip (part of core)
blocks formation into these fibres -see nothing on EM
same thing seen w first 6 N-terminal AAs (of core -past the disordered region)

tips of the a-helical core are important for self-assembly

Question 9

SYCP3 building block complex

Answer 9

forms antiparralel tetramers
2 chains in each direction
3 chain coiled coil

20nm long - close to 23nm spacing in big fibre

v stable structure

Question 10

SYCP3 and DNA

Answer 10

this tetramer binds DNA at either end
so can bind 2 DNAs at once (can see it pull down 2 differentially tagged DNAs at once

Question 11

SYCP3 chromatin compaction

Answer 11

individual 20nm tetramers organise vertically
Ends of protein outside the core mediate self assembly interactions - extre 3nm needed
forms the bands seen on EM

DNA threads through the lattice of SYCP3 tetramers
through area between them that contains the self-assembly interactions
-forms small loops on inside (facing other chromosome)
-and large loops on outside (the fuzzy material seen on SC EMs)
-conserved distance between loops - 50nm
-2 23nm repeating units per 1 loop in this model - lines up with observed distance - more evidence

Question 12

SYCP3 - human infertility

Answer 12

mutations in SYCP3 can cause human infertility

deleting ONLY C-terminal tip
-can still bind to WT proteins
-so interrupts their formation into WT SC
-poisons healthy protein pool

so are deleterious in a heterozygote as well as a homozygote
mutant copy damaging function

Question 13

central filament - SYCE2 and TEX12 subunits

Answer 13

SYCE2
>central a-helical core
>disorganised regions either side

TEX12
>C-terminal a-helical core
>disordered region at N-terminal side

these proteins form fibres together
C-terminal tip of these cores are both important - delete these and formation fails

Question 14

centrak filament properties

Answer 14

v long fibrous backbone for SC
alloes SC to elongate and maintain its structure over distance

Question 15

SYCE2/TEX12 complex for central filament

Answer 15

2:2 complex of SYCE2/TEX12
2 antiparallel TEX12
2 antiparralel SYCE2
core C termini exist at both ends

surface formed by the 2 SYCE2s tessellates with same surface on another 2:2
=4:4 complex

Question 16

central filament fibre formation

Answer 16

c terminal tips of 2:2 and 4:4 complexes tessealte together
2:2-2:2 = 2nm fibre
4:4-4:4 = 4nm fibre

these assemble tigether into 10nm ->40nm fibres
forms tope that gives lateral strength to allow SC to extend along entire chromosome

delete SYCE2
SCs that do form end up very short as no central filament support

Question 17

human infertility mutations - TEX12
difficulty of finding infertility mutations

Answer 17

v rare as affect meiosis
bringing infertility means mutation is not propagated
so mutations seen are likely novel ones
makes them hard to find in patient as need to be specifically looking

SC protein mutatiosn associate w:
>complete infertility - no gametes made
>or many miscarriages

in heterozygote infertile patient
hard to find mutation as heterozygous mutations are usually filtered out in disease mutation searches as less likely to be cause
need to know what to look for - hard in patient cases
DNA sequencing to look for mutations biases towards homozygous mutants that are deleterious
and against dominant negative phenotypes

Question 18

Transverse filament proteins

Answer 18

SYCP1
>long - ~1000 AAs
a helical coiled coil
form tetrametic structures

N-terminal core tip mediated head to head assembly of SYCP1 molecules
allows linking of SYCP1 molecules from tetramers on either side of SC

tetramer at one side
then N-tips meet head-head to link either side
forms lattice linking either side together

and SYCE3
comes in and binds SYCP1 tetramer
breaks up lattice
more SYCP1 comes in and forms more of the lattice
>basically breaks up and rescues it again in more built up manner w SYCE3 incorporated

Question 19

SYCE3 deletion vs specific WY mutation

Answer 19

in SYCE3 deletion
cant progress past step 1: remain with basic not-built up but still functional transverse filaments
not optimal but still usable

in WY mutation:
SYCE3 can come in and break up lattice
but no rescue
stuck at step 2 - broken up
no functional lattice
basically worse for cell than having no SYCE3 at all
>infertility
>complete stripping of SYCP1 from SC structure
(dominant negative)

Question 20

why is SYCE3 incorporated

Answer 20

likely cause it can bind other SC proteins
help hold structure together
(eg can help recruit central element complexes)

Question 21

SYCP1 connection of central and lateral elements (includes protein names - helps to draw a structure)

Answer 21

C terminal end of core can bind DNA
(as well as tip of disorganised region at C-term??)
this side binds the DNA looped through the SYCP3 lateral element structure

N terminus of tetramer (2 coiled coils together) bind N terminus of other tetramer at the central filament (SYCE2/TEX12 ropes) and i guess associate there (aided by SYCE3)

this other tetramer binds DNA on the lateral elements condensing the other homologue