A model depicting the function of dynamin in fission (left), and s-Mgm1 in fusion (right). Both dynamin and Mgm1 shape membranes in vivo and in vitro. Dynamin dimers assemble into a helical collar, which upon GTPase activity constrict the underlying membrane to mediate fission. We propose that s-Mgm1 forms a homo-oligomeric complex in trans to create protein bridges and ordered lattices that tether opposing membranes to support mitochondrial inner membrane cristae structures, and to also undergo a GTP-induced transition to promote fusion.