The Collaborative International Dictionary of English v.0.48:

Deposer \De*pos"er\, n.
   1. One who deposes or degrades from office.
      [1913 Webster]

   2. One who testifies or deposes; a deponent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

deposer
    n 1: a person who testifies or gives a deposition [syn:
         testifier, deponent, deposer]