Final answer:
In the class I MHC pathway of antigen presentation, cytoplasmic proteins are tagged for degradation with ubiquitin, processed by proteasomes, and then presented on the cell surface after binding with MHC class I molecules.
Step-by-step explanation:
In the class I MHC pathway of antigen presentation, cytoplasmic proteins are tagged for proteolytic degradation by covalent linkage with ubiquitin. These proteins are then degraded by the proteasomes, an enzyme complex responsible for degradation and processing of proteins.
The resulting peptides are then transported into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system. There, they bind to MHC class I molecules and are presented on the cell surface.
This is vital for the immune system to recognize and destroy cells infected with intracellular pathogens such as viruses.
Cytoplasmic proteins that are bound to ubiquitin are recognized by proteasomes where they are subsequently degraded into peptide fragments. These fragments then interact with MHC class I molecules inside the cell, a critical step for the immune response against infected cells.
Professional Antigen-presenting Cells (APCs) will then display these peptides on their surface to stimulate a cytotoxic T cell immune response, signaling that the infected cell should be destroyed.