Final answer:
Calnexin has a transmembrane helix while Calreticulin does not; it contains a signal sequence instead. Understanding the insertion of transmembrane proteins like glycophorin A, which has a hydrophobic alpha helix, is crucial in cell biology.
Step-by-step explanation:
The protein Calnexin has a transmembrane (TM) helix whereas Calreticulin does not. Calreticulin, unlike Calnexin, possesses a signal sequence but lacks a TM helix. It functions in the endoplasmic reticulum lumen. In cell biology, the manner in which a transmembrane protein inserts into the membrane is important as it dictates the locations of its N- and C-termini.
The N-terminal end of a plasma membrane polypeptide usually ends up exposed to the outside of the cell, and the alpha helical domains that anchor proteins in membranes are hydrophobic. An example of such a transmembrane protein is glycophorin A, which is important in preventing red blood cells from aggregating, featuring a hydrophobic trans-membrane alpha helix.