Final answer:
The hydrophobic regions in signal anchors are appropriately long to span the cell membrane, forming hydrophobic alpha-helices that interact with the membrane's phospholipid bilayer. Secondary protein structure and hydrophobic interactions play essential roles in stabilizing the membrane-spanning portion of these proteins.
Step-by-step explanation:
The hydrophobic regions in signal anchors are 18 to 24 amino acids long because they must be able to span the hydrophobic core of the phospholipid bilayer in cell membranes. These regions form hydrophobic alpha-helices that allow the protein to interact with and become embedded within the membrane. The length of these regions is typically enough to cross the bilayer once, creating a stable anchor for the protein.
Secondary protein structure such as the alpha-helix is stabilized by hydrogen bonding of the peptide backbone. The hydrophobic 'R' groups of the amino acids in the hydrophobic helices tend to aggregate away from water (hydrophobic interactions), which assists in stabilizing the membrane-spanning structure. These interactions within the cell membrane are crucial for the proper orientation and function of membrane proteins, including transport proteins that create pores for ions and polar molecules, and structural proteins that help maintain cell and tissue integrity.