Serine and threonine are the amino acids that are most likely to be phosphorylated in a protein during a signaling event. Phosphorylation is a common post-translational modification that plays a crucial role in cellular signaling and regulation. It involves the addition of a phosphate group to a protein or other molecules by enzymes called kinases.

Phosphorylation of serine and threonine residues is the most common type of phosphorylation that occurs in proteins. There are several reasons why serine and threonine are preferred targets for phosphorylation:

1. Abundance: Serine and threonine are two of the most abundant amino acids in proteins. They are often found in flexible regions of the protein structure, making them accessible to kinases.

2. Hydroxyl group: Both serine and threonine contain a hydroxyl group (-OH) on their side chains. This hydroxyl group can act as a nucleophile and readily react with the phosphate group of ATP, which is the donor of the phosphate group during phosphorylation.

3. Structural context: Serine and threonine residues are often found within specific consensus sequences that are recognized by kinases. These consensus sequences provide a recognition motif for kinases to bind and phosphorylate the target protein.

4. Functional significance: Phosphorylation of serine and threonine residues can regulate protein function in various ways. It can induce conformational changes, alter protein-protein interactions, modulate enzymatic activity, or serve as a docking site for other proteins containing phospho-specific binding domains.

Overall, the preference for serine and threonine phosphorylation in proteins is due to their abundance, the presence of a reactive hydroxyl group, their occurrence within consensus kinase recognition sequences, and the functional consequences of their phosphorylation. This preference highlights the importance of serine and threonine phosphorylation in mediating cellular signaling events and controlling protein function.