Molecule of the Month: Poly(A) Polymerase
Poly(A) polymerase adds a long tail of adenine nucleotides at the end of messenger RNA
The Tail End
Heads and Tails
Exploring the Structure
Poly(A) Polymerase (PDB entry 2q66)
Poly(A) polymerase is highly specific for ATP, but it will use this ATP to build a poly(A) tail on nearly any RNA strand. PDB entry 2q66 captures the enzyme in the middle of building a tail. The enzyme is clamped around a short stretch of poly(A) RNA, shown in yellow. An ATP molecule is shown in red, positioned perfectly by a magnesium ion, shown in green. The enzyme in this structure, however, has been mutated so that it cannot perform the reaction. Position 154 is normally an aspartate that coordinates another magnesium ion, which performs the reaction. In this structure, the aspartate has been changed to an alanine, shown in magenta, so the catalytic magnesium is missing.
You can click on the image for an interactive Jmol view. To look more closely at the structural basis of adenine recognition in the active site, take a look at the page at Proteopedia.
Topics for Further Discussion
- Poly(A) polymerase is highly flexible, opening and closing while it performs its job. For instance, compare the open forms on the first page of this Molecule of the Month with the closed form on this page. Why is this flexibility important, and what structural features of the protein allow it to be flexible?
- Do you see any structural or functional similarities between this template-independent polymerase and polymerases that use templates, such as DNA polymerase and RNA polymerase?
October 2008, David Goodselldoi:10.2210/rcsb_pdb/mom_2008_10