Penrose MD Gay Collins is leading the account, which is thought to have fees of around £70,000.
Barra appointed Penrose to handle its PR in August 2003, but was acquired the following spring by MSCI. MSCI is best known for a series of international indices against which fund managers benchmark their investment performance.
Luther had handled PR for MSCI in Europe for the past five years.
Barra sells performance measurement tools to portfolio managers. MSCI says £1.5 trillion is benchmarked to its equity, fixed income and hedge fund indices.