What type of protein modification occurs in the Golgi apparatus?

The Golgi apparatus is a key organelle involved in the post-translational modification of proteins. One of the primary modifications that occurs in the Golgi is glycosylation, which involves the addition of carbohydrate groups to proteins. This process is essential for the proper folding, stability, and functionality of many proteins, particularly those that are secreted or membrane-bound.

In addition to glycosylation, phosphorylation can also occur in the Golgi apparatus. This modification involves the addition of phosphate groups to specific amino acids in a protein, which can alter the protein's function, localization, or interaction with other cellular molecules.

While processes like proteolytic cleavage and ubiquitination are important for protein regulation, they typically occur at different stages of protein processing or in different cellular compartments. Proteolytic cleavage usually happens in the endoplasmic reticulum or later stages of trafficking, while ubiquitination is associated with tagging proteins for degradation by the proteasome. Therefore, the occurrence of both glycosylation and phosphorylation in the Golgi apparatus is what makes the selected answer correct.