A joint venture including MRP Realty plans to break ground by the end of the year on a $120 million office tower in McLean after a tenant in the project agreed to take an equity stake in part of the 300,000-square-foot building.

Nonprofit government consultant LMI reached a deal Tuesday to buy the lower six floors at 7940 Jones Branch Drive as part of a planned headquarters relocation from nearby offices at 2000 Corporate Ridge, also in McLean.

With LMI’s commitment, MRP Managing Partner Bob Murphy said he will now seek financing to start construction on the project in the next three months. Terms of MRP’s stake in the building were not disclosed. Rockpoint Group is also part of the joint venture partnership.