Question

The tri-peptide sequence, RGD, is found in the extracellular matrix (ECM) protein fibronectin and serves as a recognition sequence for integrins (integral membrane proteins) in the cell wall. The integrin-RGD interaction anchors cells to the ECM.

Answer 1

Proteins that contain the Arg-Gly-Asp (RGD) attachment site, together with the integrins that serve as receptors for them, constitute a major recognition system for cell adhesion. The RGD sequence is the cell attachment site of a large number of adhesive extracellular matrix, blood, and cell surface proteins, and nearly half of the over 20 known integrins recognize this sequence in their adhesion protein ligands. Some other integrins bind to related sequences in their ligands. The integrin-binding activity of adhesion proteins can be reproduced by short synthetic peptides containing the RGD sequence. Such peptides promote cell adhesion when insolubilized onto a surface, and inhibit it when presented to cells in solution. Reagents that bind selectively to only one or a few of the RGD-directed integrins can be designed by cyclizing peptides with selected sequences around the RGD and by synthesizing RGD mimics. As the integrin-mediated cell attachment influences and regulates cell migration, growth, differentiation, and apoptosis, the RGD peptides and mimics can be used to probe integrin functions in various biological systems. Drug design based on the RGD structure may provide new treatments for diseases such as thrombosis, osteoporosis, and cancer.

B) The met charge of RGD is Negative at pH7.4.

C) When released into the cell membrane, newly synthesized integrin dimers are speculated to be found in the same "bent" conformation revealed by the structural studies described above. One school of thought claims that this bent form prevents them from interacting with their ligands, although bent forms can predominate in high-resolution EM structures of integrin bound to an ECM ligand. Therefore, at least in biochemical experiments, integrin dimers must apparently not be 'unbent' in order to prime them and allow their binding to the ECM. In cells, the priming is accomplished by a protein talin, which binds to the β tail of the integrin dimer and changes its conformation.^[9][10] The α and β integrin chains are both class-I transmembrane proteins: they pass the plasma membrane as single transmembrane alpha-helices. Unfortunately, the helices are too long, and recent studies suggest that, for integrin gpIIbIIIa, they are tilted with respect both to one another and to the plane of the membrane. Talin binding alters the angle of tilt of the β3 chain transmembrane helix in model systems and this may reflect a stage in the process of inside-out signalling which primes integrins. Moreover, talin proteins are able to dimerize and thus are thought to intervene in the clustering of integrin dimers which leads to the formation of a focal adhesion. Recently, the Kindlin-1 and Kindlin-2 proteins have also been found to interact with integrin and activate it

D) One role of cholesterol is to help give the cell membrane extra support. Cholesterol is more rigid than some of the other lipids in the membrane. As Cytochemistry.net explains, cholesterol helps to immobilize some of the lipid molecules around them. This extra rigidity makes the cell membrane stronger and makes it harder for small molecules to pass through the membrane. The presence of cholesterol allows the cell membrane to be strong enough to hold the cell together and to serve as an effective barrier to ions.

E) phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin.