Integrate PKI using smart card into PGP

Problem

Integrate PKI using smart card into PGP. PGP tools have been available for a long time. However, sending and receiving email requires that the computer store the owner's private key. This limits the use of PGP since a user may want to send emails at different computers. If the smart card is used in PGP, the problem can be solved since the smart card can store the owner's private key to be used with encryption and signature.