Final answer:
MHC I heavy chains bind with the membrane protein calnexin in the ER to ensure proper folding and assembly. Calnexin acts similarly to a chaperone, facilitating the correct folding of the heavy chains. This process is essential for the MHC I molecules before they are transported to the cell membrane for antigen presentation.
Step-by-step explanation:
When MHC I heavy chains (alpha chains) first enter the endoplasmic reticulum (ER), they bind to a membrane protein known as calnexin, which retains the partly folded heavy chains in the ER. This retention is critical as it allows the alpha chain to properly fold and assemble with beta-2 microglobulin. Without this chaperone-like protein, the MHC I heavy chains may not fold correctly or may be degraded.
These integral membrane proteins undergo several modifications, including the association with chaperones such as calnexin, to ensure correct folding and assembly before they can be trafficked to their final destination. After the proper folding and assembly, the MHC I molecules are transported via vesicles which later fuse with the Golgi apparatus. These proteins are then processed within the Golgi apparatus and transported to the cell surface where they play a critical role in presenting antigens to T cells.