Figure 2. Structural comparison of the two classes of SUN proteins in Arabidopsis. The five SUN proteins present in Arabidopsis thaliana can be sub-divided into two classes: the C-terminal SUN proteins (AtSUN1 and AtSUN2) and the PM3-type or mid-SUN proteins (AtSUN3- AtSUN5). Similar proteins of both sub-groups can also be found in other non-plant eukaryotes. The classical C-terminal SUN proteins contain at least one transmembrane domain and the highly conserved SUN domain at the C-terminus. The PM3-type SUN proteins contain three transmembrane domains, one at the N-terminus and two on the C-terminus while the SUN domain is localized in the middle of the protein. AtSUN3 and AtSUN4 also contain a highly conserved PAD domain (PM3-associated domain), the function of which is currently unknown.¹²