Who was the Soviet Head of State?

by GuyNoirPI

In McCullough's "Truman", he discribes a scene where Stalin says that Truman should chair the Potsdam conference as the only head of state present. Obviously, the UK's Head of State was the King. Who was the USSR's head of state? Why wasn't the Premier of the Soviet Union also the head of government?

Searocksandtrees