Companies look to leverage bank's 120m accounts to expand stock trading Matsui Securities is a pioneer in Japan's online brokerage sector, but trails competitors in account numbers. (Photo by Nikkei) TOKYO -- Japan Post Bank plans to acquire a roughly 20% stake in online brokerage Matsui Securities, Nikkei has learned, paving the way for a service that will let the bank's 120 million account holders trade stocks.