page_title: Anti-CRISPR
section_title: AcrIIA4
hierarchical_section_title: Anti-CRISPR / Structure / AcrIIA4
caption_attribution_description: English: Structure of AcrIIA4 obtained with the UCSF Chimera software, where its PDB file was uploaded. Different colours were assigned to the four different secondary structures found in this protein: blue for β-strands, red for α-helices, orange for the 3(10) helix and grey for loops. Originally, the PDB file contains the 20 lowest energy sequences (and thus, the most stable ones) superposed, one of which was randomly selected to create the figure.
context_page_description: Anti-CRISPR is a group of proteins found in phages, that inhibit the normal activity of CRISPR-Cas, the immune system of certain bacteria. CRISPR consists of genomic sequences that can be found in prokaryotic organisms, that come from bacteriophages that infected the bacteria beforehand, and are used to defend the cell from further viral attacks. Anti-CRISPR results from an evolutionary process occurred in phages in order to avoid having their genomes destroyed by the prokaryotic cells that they will infect.
  Before the discovery of this type of family proteins, the acquisition of mutations was the only way known that phages could use to avoid CRISPR-Cas mediated shattering, by reducing the binding affinity of the phage and CRISPR. Nonetheless, bacteria have mechanisms to retarget the mutant bacteriophage, a process that it is called "priming adaptation". So, as far as researchers currently know, anti-CRISPR is the most effective way to ensure the survival of phages throughout the infection process of bacteria.
context_section_description: The protein was solved using nuclear magnetic resonance (NMR); it contains 87 residues and its molecular weight is 10.182 kDa. AcrIIA4 contains:
  3 antiparallel β-strands (the first, from residues 16 to 19, the second, from 29 to 33, and the third, from 40 to 44) that form a β-sheet. This represents a 16,1% of the total number of amino acids, as 14 of them form the β-strands.
  3 α-helices (the first, 2–13 residues, the second, 50–59 residues, and the third, 68–85 residues).
  1 3₁₀ helix placed between the first (β1) and second (β2) β-strands, which starts at residue 22 and end in residue 25. The total helical part is composed of 40 residues, which is a 50,6% of the protein.
  Loops joining the different secondary structures.
  There is a good definition of the secondary structures, as the three α-helices are packed near the three β-strands. Strikingly, between β3 strand, α2 and α3 helices, there is a hydrophobic core, originated by a cluster of aromatic side chains which are attracted by non-covalent interactions, such as pi stacking. Moreover, as it is an acidic protein, there is a high concentration of negatively charged residues in the loops between β3 and α2, between α2 and α3, and in the first part of α3, which may play an important role in the inhibition of Cas9, as negative charges might imitate phosphates of nucleic acids.
caption: AcrIIA4 20 secuencias.jpg  Structure of AcrIIA4 obtained with the UCSF Chimera software,[12] where its PDB file was uploaded.[13] Different colours were assigned to the four different secondary structures found in this protein: blue for β-strands, red for α-helices, orange for the 310 helix, and grey for loops. Originally, the PDB file contains the 20 lowest energy sequences (and thus, the most stable ones) superposed, one of which was randomly selected to create the figure. [14]
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