
Mustain was sacked from Amanah in July after facing disciplinary action.
Amanah secretary-general Mohd Hatta Ramli in a statement had said its disciplinary board found that Mustain had violated Article 21.12 of the party’s constitution.
He then registered to become a member of PKR for the Sabak Bernam division on Aug 31.
Mustain was narrowly elected to the Sabak state seat in Selangor at the 2018 general election in a tight contest against candidates from Barisan Nasional and PAS.
He polled 6,981 votes for a 130-vote majority against Sallehuddin Mohd Iskan (BN) who polled 6,851, while Sallehen Mukhyi (PAS) polled 6,183 votes.
The Selangor state government is formed by Pakatan Harapan with 41 seats in the state assembly from PKR (19), DAP (16) and Amanah (six). The opposition is formed by Barisan Nasional (five, all Umno). There are three other members from the newly formed Pejuang party, which is awaiting registration.