Threshold signature schemes distribute secret information to several servers and make the whole system that maintains the secret information fault-tolerant. Since threshold signature schemes typically assume that the shared signing function can only be activated by a quorum number of servers. If anyone has a power to activate the signing function of servers, he can easily compute valid signatures for a specific organization without knowing the private key. S. Xu et al. proposed a general construction to build threshold signature schemes (called as server assisted threshold signatures) which provide an organization (e.g., a user) with controllability for activating his private signing function in a certain enhanced way. In this paper, we newly propose proactive secret sharing schemes which are suitable for server-assisted threshold signatures.