Inner membrane component of the type II secretion system required for the energy-dependent secretion of extracellular factors such as proteases and toxins from the periplasm. Plays a role in the complex assembly and recruits XcpY resulting in a stable complex in the inner membrane. Provides thus a link between the energy-providing XcpR protein in the cytoplasm and the rest of the T2SS machinery. Type II secretion system is composed of four main components: the outer membrane complex, the inner membrane complex, the cytoplasmic secretion ATPase and the periplasm-spanning pseudopilus (By similarity). Forms homodimers (By similarity). Interacts with XcpY/GspL (PubMed:9884230, PubMed:11872731, PubMed:16168578, PubMed:30337370). Interacts with XcpR/GspE and XcpS/GspF (PubMed:16168578). Two distinct periplasmic domains, one located in the proximity of the transmembrane domain and the other located towards the C-terminal part of the protein are both involved in the interaction with XcpY. Absence of XcpZ results in decreased amount of XcpY in type II secretion system complexes. Belongs to the GSP M family.