A sovereign institutional investor has formed a joint venture with Senior Housing Properties Trust and will contribute roughly $261 million for a 45% equity interest in two Boston life science properties the company bought in 2014.

Senior Housing Properties will continue to own the remaining 55% stake in the properties, at 11 Fan Pier and 50 Northern Ave., it said in a release. The company said it is subject to a confidentiality agreement and cannot disclose the identity of the joint venture partner.

The joint venture deal values the buildings at $1.2 billion; the company acquired them in May 2014 for roughly $1.13 billion.

The assets, 15-story class A life science properties in Boston's Seaport District, are 95% leased to Vertex Pharmaceuticals Inc. through 2028. They include 1.1 million rentable square feet of lab, corporate office and street-level retail space.

Senior Housing Properties expects to use the cash proceeds from the deal to partly repay outstanding amounts under its revolving credit facility.