複選題6. Which statement(s) is suitable to describe ubiquitin? (A) An isopeptide bond forms between the C-terminus of ubiquitin and the e-amino group of the modified lysine residue. (B) Ubiquitination always leads the modified proteins to degradation. (C) ATP is required for activation of ubiquitin. (D) Multiple ubiquitin molecules can be covalently linked together to form a polyubiquitin chain on the modified protein. (E) The N-terminal amino acid of the modified protein is one determinant of its half-life.