Microsoft Corp. once again surpassed $4 trillion in valuation, joining artificial intelligence chipmaker Nvidia in the exclusive club that also briefly included Apple on Tuesday. The sky-high valuations highlight the investor frenzy around artificial intelligence.

Earlier in the day, OpenAI said it has reorganized its ownership structure and converted its business into a public benefit corporation after two crucial regulators, the Delaware and California attorneys general, said they would not oppose the plan. It also said has signed a new agreement with its longtime backer Microsoft that gives the software giant a roughly 27% stake in OpenAI's new for-profit corporation.

The news gave Microsoft's shares a boost as the stock closed up 2% at $542.07, valuing the technology giant at $4.04 trillion. Microsoft's valuation previously passed $4 trillion in July, making it the second company after Nvidia to reach the milestone.

Apple's shares, meanwhile, crossed the $4 trillion line earlier Tuesday before closing up slightly at $269 and a total valuation of $3.99 trillion. Thanks to the iPhone's success, Apple was the the first publicly traded company to valued at $1 trillion, $2 trillion and eventually, $3 trillion.