Jayson Tatum will be a Celtic through his prime

Jayson Tatum will be a Celtic through his prime after reportedly signing a five-year mega-extension on Monday, the largest in NBA history

Why it matters: Tatum, 26, is half of a Celtics core that's likely to compete for NBA titles for years to come — along with Finals MVP Jaylen Brown, whose own $304-million extension last year was record-breaking.

Zoom in: Tatum and the team agreed to a five-year, $315-million extension, per TNT and Bleacher Report's Chris Haynes .

• Tatum's deal is known as a "supermax," referring to a provision in NBA rules that lets veterans who've achieved certain accolades be paid as much as 35% of the salary cap.

Other business: While other Eastern Conference contenders are adding players to challenge Boston, the Celtics have been busy retaining key contributors to their title-winning team.

• Guard Derrick White agreed to a four-year contract extension worth $126 million, per the Globe .