複選題30. For the following post-translational modification statements, which is (are) CORRECT? (A) Ubiquitin modifies proteins at their arginine residues. (B) There are several specific types of ubiquitin linkages; among them, the linkage at the 48th position (K48) for polyubiquitination is a well-known protein degradation signal. (C) Phosphorylation on proteins is found on three amino acid residues: serine, threonine, and tyrosine. (D) Lysine often serves as a phosphomimic residue to mimic a constitutive phosphorylation condition. (E) Ubiquitination requires three enzymes-E1, E2, and E3-to achieve.