Ubuntu: NO PUBKEY for www.openprinting.org

• Description:
  when running apt-get update, got the following error:

  W: GPG error: http://www.openprinting.org lsb3.2 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7A4B44C2D2A2203E

• Solution:
  

  sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7A4B44C2D2A2203E
  sudo apt-get update