H3 Toth- Animal extracellular matrix and plant cell wall

what are the different components of the animal ECM and their functions?

fibrils are made from collagen and elastin proteins:

• provide strength/stiffness/elasticity

• protect against tension and compression

• insoluble

fibril crosslinkers are made from accessory collagen proteins:

• organise 3D fibril network

• strengthen the network

gel is made from glycosaminoglycans (GAGs) polysaccharide and proteoglycans (GAGs attached to a protein core):

• contains the fibril network embedded

• creates a hydrophilic environment = hydration

• protects against compression

contains protein components (unlike the plant cell wall)

describe the structure of collagen

• collagen is an insoluble fibrous protein that makes up the fibrils of the animal ECM

• it is a right handed triple helix of three peptide strands made up of three amino acid repeats, every 3rd aa. being glycine

• glycine is the smallest amino acid, so when the collagen forms a triple helix, the glycine is at the centre to make the most compact possible shape

• rich in proline, lysine and hydroxyproline

what are the steps of collagen synthesis?

• ER: synthesis of pro α-chains (precursors)

• ER: proline and lysine hydroxylation- vitamin C is a cofactor

• ER: glycosylation of hydroxylysines

• ER: self-assembly of the chains into a procollagen triple helix

• golgi: N-linked glycan modifications for recognition

• secretory vesicles: transfer to plasma membrane

• ECM: cleavage of the ends by extracellular proteases

• ECM: self-assembly into collagen fibril

• ECM: aggregation of fibrils into a fibre

collagen fibres cross-link at specific, regular points, giving them a striated/banded pattern

what are the two accessory forms of collagen and their structures?

• crosslinker collagen has a trihelix structure interrupted by one or two hinge regions, which allow cross-linking (determines the ECM thickness)

• network-forming collagen has more regular non-helical breaks, so they are very flexible and can assemble into a mesh sheet, which can be stacked by interactions between N and C globular domains to form the basal laminae

what is the basal laminae and what does it do?

• the basal laminae is a specialised form of ECM that lies underneath epithelial cells and surrounds muscle, fat, and schwann cells to facilitate anchorage to connective tissue

• this is primarily made up of layers of network-forming collagen sheets, with laminin glycoproteins

this has multiple regulatory roles, eg:

• determining cell polarity if needed

• filtering intake of molecules

• organise proteins in adjacent plasma membrane

• induce cell differentiation

• serve as “highways” for cell migration

what is the structure of elastin

• elastin is an insoluble protein network that forms part of the fibrils of the animal ECM

what is the general structure and properties of glycosaminoglycans?

• GAGs are polymers of disaccharide monomers, which make up most of the gel in the animal ECM

• one monomer is a -uronic acid, the other is an amino sugar

• the most abundant is hyaluronic acid, which is glucuronic acid + N-acetylglucosamine

• these are strongly hydrophilic and water-soluble

• they are negatively charged due to high amounts of sulfur, which attracts sodium- this affects osmosis to give turgor pressure

• they are inflexible and have a very low density

what is the general structure and properties of proteoglycans?

• proteoglycans form part of the animal ECM

• these are proteins highly modified by O-linked glycosylation, with many glycosaminoglycans (GAGs) attached via serine or threonine residues (in the golgi)

what are the different components of the plant cell wall and their functions?

• very little protein content (unlike animal ECM)

describe the organisation of cellulose fibers in the plant cell wall

• cellulose polymers are produced right next to each other so that they form hydrogen bonds together

• this produces a long sheet of adjacent polymer molecules, which is hydrophilic at the edges but hydrophobic on either face

• these stack into microfibrils, which have a crystalline core, that can assemble into fibrils due to having a hydrophobic faces on the top and bottom and a hydrophilic faces on either side

how is cellulose deposition in the plant cell wall regulated?

• cellulose is synthesised by terminal rosette enzyme complexes in the plasma membrane (acquired by HGT from bacteria)

• the terminal rosettes contain 6 particles, each of which contain 3-6 cellulose synthase A (CESA) proteins, which each make one cellulose polymer at a time

• this means each rosette makes an 18-24 chain microfibril and twists them together

• the cell elongates perpendicular to the orientation of the cellulose fibrils, because new material is deposited in between the fibrils

• random cellulose deposition in a cell would mean it could never expand

• the deposition of cellulose by terminal rosettes is directed by microtubules to occur in one direction

• plant cells stop growing by depositing new layers in different orientations

• this deposition is driven by the polymerisation reaction because the polymer is being pushed into an already densely packed matrix, so the terminal rosette is the part that moves instead, along the microtubules

describe the general structure and function of hemicellulose, with an example

• hemicellulose cross links the cellulose fibrils in the plant cell membrane

• ‘half’ cellulose because one side of the chain has many glucan side chains, while the other can interact with cellulose to form mechanical hot spots

• eg. xyloglucan

describe the composition and function of the gel in plant cell walls

• the gel is mostly made up of pectins (very complex, hydrophilic, acidic polysaccharides), with its consistency regulated by pH and Ca ions

• these protect against pathogens and influence the porosity, pH, and ion balance of the cell wall

• the gel can also contain lignin (polyphenol), primarily in the secondary cell wall (more rigid, produced by mature cells)

how do animal cells anchor with the ECM?

• animal cells have integrins, transmembrane protein receptors that mediate cell adhesion to the ECM

• these are heterodimeric with one alpha and one beta chain, which can bind to different ECM components at regions called focal adhesions

• these focal adhesions define cell shape, mechanical and chemical signalling and control cell migration

how does the cell modify the cell wall for growth?

• cell growth requires the loosening of the cell wall crosslinking, and the increase of turgor pressure, according to the lockhart equation: R = ɸ(P-Y)

• cell expansion happens when P, pressure > Y, the yield threshold

• this is determined by the wall extensibility, ɸ (determined by cross linking)

• the cross linking is weakened by acidic extracellular pHs (< 5), caused by hormones eg. auxins

• this triggers expansin proteins to disrupt H bonds between cellulose and hemicellulose and reduce pectin rigidity

• local cell wall loosening at the apical meristem initiates new organ formation

• it can also be used to detect abiotic/biotic stress (eg. pectin degradation by pathogens)