(Adds details, fund comments from paragraph 3 onwards)

TOKYO, Nov 14 (Reuters) - Artisan Partners, a U.S.-based investor in Japanese retail giant Seven & i Holdings , called on the company's special committee to consider a bidding process of competing takeover proposals to secure the highest offer.

The 7-Eleven convenience store owner received a buyout proposal from a member of its founding Ito family, it said on Wednesday, a potential $58 billion white-knight bid as it weighs a rival offer from Canada's Alimentation Couche-Tard .

The offer from Ito-Kogyo, a company linked to Seven & i Vice President Junro Ito, is non-binding and under review by the same special committee set up to assess Couche-Tard's bid.

In a statement, Artisan portfolio manager Ben Herrick said the fund supports both offers at this stage and urged the committee to consider a formal bidding process, including an auction, to explore additional third-party interest.

"Furthermore, we strongly recommend that the board grant both parties equal access to conduct due diligence," Herrick said. "Lastly, it is imperative for the board and special committee to act with a sense of urgency without further delay."

Artisan holds 1.11% of Seven & i shares, according to LSEG data. The fund is among Seven & i's vocal foreign investors that have urged the company to focus on its core convenience store business.

(Reporting by Rocky Swift; Editing by Kim Coghill and Jamie Freed)

((rocky.swift@thomsonreuters.com;))