Proton Mail can sign (and encrypt) emails when it knows the PGP key of the correspondent. For this is provides PGP keys to its users.
Unfortunately most of them are not searchable via the traditional PGP keyservers. With the following command you can download the public PGP key of a Proton Mail user:

curl -s 'https://api.protonmail.ch/pks/lookup?op=get&search=user@protonmail.com'

Also, just discovered that GPG Keychain on Mac detects if the clipboard contains a PGP key and asks if it should import it. Very nice feature which saves a couple clicks :-)