DALLAS — The Mt. Gox bitcoin exchange that recently collapsed in Japan has filed for U.S. bankruptcy protection to shield itself from a lawsuit seeking to repay thousands of people whose digital currency is now missing.The U.S. filing made in Dallas late Sunday supplements a bankruptcy petition that Mt. Gox submitted in Japan at the end of last month.Mt. Gox was once the world’s largest exchange specializing in bitcoins, but now finds itself in a financial mess after losing about 850,000 bitcoins valued at $473 million, according to court documents.Although it’s based in Tokyo, Mt. Gox is opening a bankruptcy case in the U.S. in an attempt to delay a recent federal lawsuit filed in Illinois on behalf of all U.S. residents burned by the exchange’s abrupt demise.