Licensed Shared Access (LSA) is a new complementary regulatory framework which aims to optimize wireless spectrum utilization. Under LSA, an incumbent can share his owned spectrum through an "LSA licensee". This sharing is supervised by the regulator, but the allocation and pricing mechanism to be used is not specified yet. To fill that gap, in this paper we propose PAM: Proportional Allocation Mechanism, which is a truthful auction mechanism offering a good compromise between fairness and efficiency and can generate the highest revenue to the regulator compared to other truthful mechanisms proposed in the literature.