Apt-get update error: http://extras.ubuntu.com Public key unavailable

GPG error: http://extras.ubuntu.com
natty Release: The following
signatures couldn't be verified
because the public key is not
available: NO_PUBKEY 16126D3A3E5C1192

I think I selected to install things from extras.ubuntu.com when I used the expert mode installer, but I got the above error when doing apt-get update. How do I fix this?

gpg --keyserver keyserver.ubuntu.com --recv 3E5C1192
gpg --export --armor 3E5C1192 | sudo apt-key add -
sudo apt-get update

This could be a solution. From

http://www.ubuntugeek.com/fix-for-gpg-error-httpextras-ubuntu-com-maverick-release.html

You can also get all launchpad keys automatically using the below method

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install launchpad-getkeys
sudo launchpad-getkeys

From

Ubuntu Extras keyring error