Final answer:
Transmembrane proteins contain alpha-helices with hydrophobic amino acids allowing them to span the cell membrane and have various functions including cell adhesion and transport. The N-terminal signal and stop-transfer sequences dictate their membrane orientation and multi-spanning nature. Glycophorin A is an example of such a protein.
Step-by-step explanation:
Transmembrane proteins typically contain one or more transmembrane helices, which are alpha-helices that span the cell membrane. These helices are primarily composed of hydrophobic amino acids, allowing them to interact with the hydrophobic interior of the lipid bilayer. Transmembrane proteins often have multiple functions, including cell adhesion and the formation of pores for the transport of molecules and ions. The synthesis of these proteins involves specific sequences that guide their insertion into the membrane. For instance, the N-terminal end of such a protein will often end up outside the cell following synthesis, due to the presence of an N-terminal signal sequence. Additionally, stop-transfer sequences within the protein can cause it to span the membrane multiple times if multiple such sequences are present. Glycophorin A is an example of a transmembrane protein that prevents red blood cells from clumping in circulation, with a single hydrophobic transmembrane alpha helix.