By Suzanne McGee

(Reuters) - The State of Michigan Retirement System, which oversees some $143.9 million in pension fund assets for state employees, has invested $6.6 million in the ARK 21Shares Bitcoin ETF, it disclosed in a regulatory filing on Friday.

This marks the second confirmed investment by a pension fund in the spot bitcoin exchange-traded funds (ETFs) launched in January, a sign the assets are slowly beginning to attract traditional institutional investors in a boost for crypto assets.

In May, the state of Wisconsin's investment board, which manages $156 billion in assets for the Wisconsin Retirement System, revealed holdings of BlackRock's iShares Bitcoin Trust and a stake in the Grayscale Bitcoin Trust.

Those holdings were valued at $99 million and $63 million, respectively, as of the end of March. That pension fund has yet to file its second-quarter portfolio update.

The quarterly disclosures, known as 13-F filings, are made to the Securities and Exchange Commission around 45 days after the end of each quarter and may not reflect current positions.

While retail investors have accounted for the lion's share of the $32 billion that has poured into the nine newly-launched bitcoin ETFs over the last six months, according to VettaFi, analysts are watching for signs of any uptick in demand on the part of institutions like the Michigan pension fund.

"Institutions allocating to these funds are likely to have a longer time horizon," said Todd Sohn, an ETF analyst at Strategas. A greater preponderance of long-term investors should reduce the often-extreme volatility of bitcoin, other analysts say.

Separately, the mayor of Jersey City, New Jersey, said on the social media platform X that it plans to make an allocation to the bitcoin ETFs, without providing a timeframe.

The State of Michigan Retirement System did not respond to a request for comment.

(Reporting by Suzanne McGee; Editing by Michelle Price and Marguerita Choy)