Crystal structure of the human CCA-adding enzyme: insights into template-independent polymerization

J Mol Biol. 2003 May 16;328(5):985-94. doi: 10.1016/s0022-2836(03)00381-4.


All tRNA molecules carry the invariant sequence CCA at their 3'-terminus for amino acid attachment. The post-transcriptional addition of CCA is carried out by ATP(CTP):tRNA nucleotidyltransferase, also called CCase. This enzyme catalyses a unique template-independent but sequence-specific nucleotide polymerization reaction. In order to reveal the molecular mechanism of this activity, we solved the crystal structure of human CCase by single isomorphous replacement. The structure reveals a four domain architecture with a cluster of conserved residues forming a positively charged cleft between the first two domains. Structural homology of the N-terminal CCase domain to other nucleotidyltransferases could be exploited for modeling a tRNA-substrate complex. The model places the tRNA 3'-end into the N-terminal nucleotidyltransferase site, close to a patch of conserved residues that provide the binding sites for CTP and ATP. Based on our results, we introduce a corkscrew model for CCA addition that includes a fixed active site and a traveling tRNA-binding region formed by flexible parts of the protein.

MeSH terms

  • Amino Acid Sequence
  • Catalytic Domain
  • Conserved Sequence
  • Crystallography, X-Ray
  • Dimerization
  • Humans
  • In Vitro Techniques
  • Models, Biological
  • Models, Molecular
  • Molecular Sequence Data
  • Protein Structure, Quaternary
  • Protein Structure, Tertiary
  • RNA Nucleotidyltransferases / chemistry*
  • RNA Nucleotidyltransferases / genetics
  • RNA Nucleotidyltransferases / metabolism
  • RNA Processing, Post-Transcriptional
  • RNA, Transfer / genetics
  • RNA, Transfer / metabolism
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Static Electricity
  • Substrate Specificity


  • Recombinant Proteins
  • RNA, Transfer
  • RNA Nucleotidyltransferases
  • tRNA nucleotidyltransferase

Associated data

  • PDB/1OU5